When using FxFx merging, the number of events in the final output (e.g. HepMC) is smaller than the number of events requested in the run_card

> I am trying to generate **Z+1j** events using
> `generate p p > mu+ mu- [QCD] @1` and
> `add process p p > mu+ mu- j [QCD] @2`.
> I am using the **FxFx merging** scheme, but only about **5800 out of 10000** events are produced.
> I attach the **run card** and **shower card** here for reference.
#***********************************************************************
# MadGraph5_aMC@NLO *
# *
# run_card.dat aMC@NLO *
# *
# This file is used to set the parameters of the run. *
# *
# Some notation/conventions: *
# *
# Lines starting with a hash (#) are info or comments *
# *
# mind the format: value = variable ! comment *
# *
# Some of the values of variables can be list. These can either be *
# comma or space separated. *
# *
# To display additional parameter, you can use the command: *
# update to_full *
#***********************************************************************
#
#*******************
# Running parameters
#*******************
#
#***********************************************************************
# Tag name for the run (one word) *
#***********************************************************************
  tag_1 = run_tag ! name of the run
#***********************************************************************
# Number of LHE events (and their normalization) and the required *
# (relative) accuracy on the Xsec. *
# These values are ignored for fixed order runs *
#***********************************************************************
  10000 = nevents ! Number of unweighted events requested
  -1.0 = req_acc ! Required accuracy (-1=auto determined from nevents)
  -1 = nevt_job ! Max number of events per job in event generation.
                 ! (-1= no split).
#***********************************************************************
# Output format
#***********************************************************************
  -1.0 = time_of_flight ! threshold (in mm) below which the invariant livetime is not written (-1 means not written)
  average = event_norm ! average/sum/bias. Normalization of the weight in the LHEF
#***********************************************************************
# Number of points per itegration channel (ignored for aMC@NLO runs) *
#***********************************************************************
  0.01 = req_acc_fo ! Required accuracy (-1=ignored, and use the
                     ! number of points and iter. below)
# These numbers are ignored except if req_acc_FO is equal to -1
  5000 = npoints_fo_grid ! number of points to setup grids
  4 = niters_fo_grid ! number of iter. to setup grids
  10000 = npoints_fo ! number of points to compute Xsec
  6 = niters_fo ! number of iter. to compute Xsec
#***********************************************************************
# Random number seed *
#***********************************************************************
  0 = iseed ! rnd seed (0=assigned automatically=default))
#***********************************************************************
# Collider type and energy *
# 0 = no PDF *
# 1/-1 = proton/antiproton *
# 3/-3 = electron/positron with ISR/Beamstrahlung; *
# 4/-4 = muon/antimuon with ISR/Beamstrahlung; *
#***********************************************************************
  1 = lpp1 ! beam 1 type (0 = no PDF)
  1 = lpp2 ! beam 2 type (0 = no PDF)
  6500.0 = ebeam1 ! beam 1 energy in GeV
  6500.0 = ebeam2 ! beam 2 energy in GeV
#***********************************************************************
# PDF choice: this automatically fixes also alpha_s(MZ) and its evol. *
#***********************************************************************
  nn23nlo = pdlabel ! PDF set
  244600 = lhaid ! If pdlabel=lhapdf, this is the lhapdf number. Only
              ! numbers for central PDF sets are allowed. Can be a list;
              ! PDF sets beyond the first are included via reweighting.
  0 = pdfscheme ! the scheme of the input PDFs. 0->MSbar; 1->DIS
             ! 2->eta (leptonic); 3->beta (leptonic)
      ! 4->mixed (leptonic); 6->delta (leptonic)
                    ! if pdlabel==emela, this is set automatically
#***********************************************************************
# The following block is specific to lepton collisions (lpp=+-3) *
#***********************************************************************
  True = photons_from_lepton ! whether to include or not photons from
                              ! lepton ISR
#***********************************************************************
# Include the NLO Monte Carlo subtr. terms for the following parton *
# shower (HERWIG6 | HERWIGPP | PYTHIA6Q | PYTHIA6PT | PYTHIA8) *
# WARNING: PYTHIA6PT works only for processes without FSR!!!! *
#***********************************************************************
  PYTHIA8 = parton_shower
  1.0 = shower_scale_factor ! multiply default shower starting
                            ! scale by this factor
  False = mcatnlo_delta ! use MC@NLO-Delta matching, arXiv:2002.12716
                        ! (only with Pythia8309 or later)
#***********************************************************************
# Renormalization and factorization scales *
# (Default functional form for the non-fixed scales is the sum of *
# the transverse masses divided by two of all final state particles *
# and partons. This can be changed in SubProcesses/set_scales.f or via *
# dynamical_scale_choice option) *
#***********************************************************************
  False = fixed_ren_scale ! if .true. use fixed ren scale
  False = fixed_fac_scale ! if .true. use fixed fac scale
  False = fixed_QES_scale
  91.118 = mur_ref_fixed ! fixed ren reference scale
  91.118 = muf_ref_fixed ! fixed fact reference scale
  -1 = dynamical_scale_choice ! Choose one (or more) of the predefined
           ! dynamical choices. Can be a list; scale choices beyond the
           ! first are included via reweighting
  1.0 = mur_over_ref ! ratio of current muR over reference muR
  1.0 = muf_over_ref ! ratio of current muF over reference muF

#***********************************************************************
# Reweight variables for scale dependence and PDF uncertainty *
#***********************************************************************
  1.0, 2.0, 0.5 = rw_rscale ! muR factors to be included by reweighting
  1.0, 2.0, 0.5 = rw_fscale ! muF factors to be included by reweighting
  True = reweight_scale ! Reweight to get scale variation using the
            ! rw_rscale and rw_fscale factors. Should be a list of
            ! booleans of equal length to dynamical_scale_choice to
            ! specify for which choice to include scale dependence.
  False = reweight_pdf ! Reweight to get PDF uncertainty. Should be a
            ! list booleans of equal length to lhaid to specify for
            ! which PDF set to include the uncertainties.
#***********************************************************************
# Store reweight information in the LHE file for off-line model- *
# parameter reweighting at NLO+PS accuracy *
#***********************************************************************
  False = store_rwgt_info ! Store info for reweighting in LHE file
#***********************************************************************
# Customization of the code. List of files containing user hook function
#***********************************************************************
   = custom_fcts ! List of files containing user hook function
#***********************************************************************
# ickkw parameter: *
# 0: No merging *
# 3: FxFx Merging - WARNING! Applies merging only at the hard-event *
# level. After showering an MLM-type merging should be applied as *
# well. See http://amcatnlo.cern.ch/FxFx_merging.htm for details. *
# 4: UNLOPS merging (with pythia8 only). No interface from within *
# MG5_aMC available, but available in Pythia8. *
# -1: NNLL+NLO jet-veto computation. See arxiv:1412.8408 [hep-ph]. *
#***********************************************************************
  3 = ickkw
#***********************************************************************
#
#***********************************************************************
# BW cutoff (M+/-bwcutoff*Gamma). Determines which resonances are *
# written in the LHE event file *
#***********************************************************************
  15.0 = bwcutoff
#***********************************************************************
# Cuts on the jets. Jet clustering is performed by FastJet. *
# - If gamma_is_j, photons are also clustered with jets. *
# Otherwise, they will be treated as tagged particles and photon *
# isolation will be applied. Note that photons in the real emission *
# will always be clustered with QCD partons. *
# - When matching to a parton shower, these generation cuts should be *
# considerably softer than the analysis cuts. *
# - More specific cuts can be specified in SubProcesses/cuts.f *
#***********************************************************************
  1.0 = jetalgo ! FastJet jet algorithm (1=kT, 0=C/A, -1=anti-kT)
  1.0 = jetradius ! The radius parameter for the jet algorithm
  9.0 = ptj ! Min jet transverse momentum
  -1.0 = etaj ! Max jet abs(pseudo-rap) (a value .lt.0 means no cut)
  False = gamma_is_j ! Wether to cluster photons as jets or not
#***********************************************************************
# Cuts on the charged leptons (e+, e-, mu+, mu-, tau+ and tau-) *
# More specific cuts can be specified in SubProcesses/cuts.f *
#***********************************************************************
  0.0 = ptl ! Min lepton transverse momentum
  -1.0 = etal ! Max lepton abs(pseudo-rap) (a value .lt.0 means no cut)
  0.0 = drll ! Min distance between opposite sign lepton pairs
  0.0 = drll_sf ! Min distance between opp. sign same-flavor lepton pairs
  0.0 = mll ! Min inv. mass of all opposite sign lepton pairs
  30.0 = mll_sf ! Min inv. mass of all opp. sign same-flavor lepton pairs
#***********************************************************************
# Fermion-photon recombination parameters *
# If Rphreco=0, no recombination is performed *
#***********************************************************************
  0.1 = rphreco ! Minimum fermion-photon distance for recombination
  -1.0 = etaphreco ! Maximum abs(pseudo-rap) for photons to be recombined (a value .lt.0 means no cut)
  False = lepphreco ! Recombine photons and leptons together
  False = quarkphreco ! Recombine photons and quarks together
#***********************************************************************
# Photon-isolation cuts, according to hep-ph/9801442 *
# Not applied if gamma_is_j *
# When ptgmin=0, all the other parameters are ignored *
# More specific cuts can be specified in SubProcesses/cuts.f *
#***********************************************************************
  20.0 = ptgmin ! Min photon transverse momentum
  -1.0 = etagamma ! Max photon abs(pseudo-rap)
  0.4 = r0gamma ! Radius of isolation code
  1.0 = xn ! n parameter of eq.(3.4) in hep-ph/9801442
  1.0 = epsgamma ! epsilon_gamma parameter of eq.(3.4) in hep-ph/9801442
  True = isoem ! isolate photons from EM energy (photons and leptons)
#***********************************************************************
# Cuts associated to MASSIVE particles identified by their PDG codes. *
# All cuts are applied to both particles and anti-particles, so use *
# POSITIVE PDG CODES only. Example of the syntax is {6 : 100} or *
# {6:100, 25:200} for multiple particles *
#***********************************************************************
  {} = pt_min_pdg ! Min pT for a massive particle
  {} = pt_max_pdg ! Max pT for a massive particle
  {} = mxx_min_pdg ! inv. mass for any pair of (anti)particles
#***********************************************************************
# Use PineAPPL to generate PDF-independent fast-interpolation grid *
# (https://zenodo.org/record/3992765#.X2EWy5MzbVo) *
#***********************************************************************
  False = pineappl ! PineAPPL switch
#***********************************************************************
# Folding parameters for S-events to reduce the number of negatively *
# weighted events. Allowed values are 1, 2, 4 or 8 for each of the *
# three variables. Typically, folding in xi_i or y_ij results in the *
# largest reduction of negatively weighted events. (arXiv:2002.12716) *
#***********************************************************************
  1, 1, 1 = folding ! correspond to folding in xi_i, y_ij, and phi_i
#***********************************************************************

#***********************************************************************
# MadGraph5_aMC@NLO *
# *
# shower_card.dat aMC@NLO *
# *
# This file is used to set the parameters for the shower. *
# *
# Some notation/conventions: *
# *
# Lines starting with a hash (#) are info or comments *
# *
# mind the format: variable = value # comment *
#***********************************************************************
#
#***********************************************************************
# Shower settings *
#***********************************************************************
# Number of events, jobs, errors, and random seeds *
#***********************************************************************
nevents = -1 # N evts to shower (< 0 = all)
nsplit_jobs = 1 # N jobs to run in parallel (< 100!!)
combine_td = T # combine the topdrawer/HwU files if nsplit_jobs>1
maxprint = 2 # N evts to print in the log
maxerrs = 0.1 # max fraction of errors
rnd_seed = 0 # 1st random seed (0 = default)
rnd_seed2 = 0 # 2nd random seed (0 = default) !ONLY FOR HWERIG6!
#***********************************************************************
# PDFs and non-perturbative modelling *
#***********************************************************************
pdfcode = 1 # 0 = internal, 1 = same as NLO, other = lhaglue
ue_enabled = F # underlying event
hadronize = T # hadronisation on/off !IGNORED BY HERWIG6!
lambda_5 = -1.0 # Lambda_5 (< 0 = default) !IGNORED BY PYTHIA8!
#***********************************************************************
# Stable or unstable particles *
#***********************************************************************
b_stable = F # set B hadrons stable
pi_stable = T # set pi0's stable
wp_stable = F # set w+'s stable
wm_stable = F # set w-'s stable
z_stable = F # set z0's stable
h_stable = F # set Higgs' stable
tap_stable = F # set tau+'s stable
tam_stable = F # set tau-'s stable
mup_stable = F # set mu+'s stable
mum_stable = F # set mu-'s stable
#***********************************************************************
# Mass of the b quark *
#***********************************************************************
b_mass = -1.0 # if < 0 = read from SubProcesses/MCmasses_*.inc
#***********************************************************************
# Special settings *
#***********************************************************************
is_4lep = F # T if 4-lepton production !ONLY FOR PYTHIA6!
is_bbar = F # T if bb~ production !ONLY FOR HERWIG6!
#***********************************************************************
# Pythia8 Specific Options (FxFx and advanced options)
#***********************************************************************
Qcut = 20.0 # Merging scale for FxFx
njmax = 1 # Maximal multiplicity in the merging (fxFx).
# # -1 means guessed from the process definition
qed_shower = T # T = enable QED shower for Q and L
primordialkt = F # T = enable primordial parton k_T
pythia8_options = {} # Dictionary specifying any additional entry in the pythia8 command file
                       # Warning changing some parameter will break NLO accuracy.
#***********************************************************************
# Matrix-element corrections !ONLY FOR PYTHIA8!
#***********************************************************************
space_shower_me_corrections = F # MECs for ISR
time_shower_me_corrections = T # MECs for FSR
time_shower_me_extended = F # see Pythia8 manual as well as
time_shower_me_after_first = F # hep-ph/2308.06389 for details
#***********************************************************************
# Decay channels *
#***********************************************************************
# Syntax for HERWIG6 *
# DM_I = M > D1 D2 @ BR @ ME *
# corresponding to call to HWMODK(M,BR,ME,D1,D2) *
# I < 100, M is the decaying resonance, D1, D2, ... are the decay *
# products (up to five), BR is the branching ratio and ME is the type *
# of matrix element to be used in the decay. *
# BR's are correctly understood only if they add up to 1, and only if *
# no more than three modes are required for a given resonance. *
# WARNING: the order of decay products in > 2-body decays IS RELEVANT. *
# *
# Syntax for PYTHIA6 *
# DM_I = M > D1 D2 @ BR @ ME *
# WARNING: turning hadronisation off disables top decays *
# WARNING: 1 -> n decays (with n > 2) are handled through a sequence *
# of 1 -> 2 decays. *
# WARNING: entries BR and ME are ignored *
# *
# Syntax for HERWIG++ *
# DM_I = M > D1 D2 @ BR @ ME *
# WARNING: entries BR and ME are ignored *
# *
# Syntax for PYTHIA8 *
# DM_I = M:onIfAny = D1 D2 *
# or similar, according to the offical PYTHIA8 decay syntax, see *
# the online PYTHIA8 manual *
# WARNING: 1 -> n decays (with n > 2) are handled through a sequence *
# of 1 -> 2 decays. *
# *
# Examples *
# Z -> e+ e- or mu+ mu- with BR = 0.5 each, HERWIG6 *
# DM_1 = 23 > -11 11 @ 0.5d0 @ 100
# DM_2 = 23 > -13 13 @ 0.5d0 @ 100
# H -> ta+ ta- with BR = 1, HERWIG6 or HERWIG++ *
# DM_3 = 25 > -15 15 @ 1.0d0 @ 0
# t -> ve e+ b with BR = 1, HERWIG6 or HERWIG++ *
# DM_4 = 6 > 12 -11 5 @ 1d0 @ 100
# t -> ve e+ b with BR = 1, PYTHIA6 *
# DM_5 = 6 > 24 5 @ 1d0 @ 100
# DM_6 = 24 > 12 -11 @ 1d0 @ 100
# W+ -> ve e+, W- -> vm~ mu-, PYTHIA8 *
# DM_1 = 24:onMode = off
# DM_2 = 24:onPosIfAny = 11 12
# DM_3 = 24:onNegIfAny = 13 14
# W+ -> ve e+, W- -> ve~ e- and vm~ mu-, PYTHIA8 *
# DM_1 = 24:onMode = off
# DM_2 = 24:onIfAny = 11 12
# DM_3 = 24:onNegIfAny = 13 14
#***********************************************************************
# Extra libraries/analyses *
#***********************************************************************
# The following lines need to be changed if the user does not want to *
# create a StdHEP/HepMC file, but to directly run an own analysis (to *
# be placed in HWAnalyzer or analogous MCatNLO subfolders). *
# Please use files in those folders as examples. *
# Remember that if your analysis uses hbook or is in the HwU format, *
# you must also add to hbook.o or HwU.o to the ANALYSE list as well. *
#***********************************************************************
EXTRALIBS = pythia8 z dl stdc++ # Extra-libraries (not LHAPDF)
                                 # Default: "stdhep Fmcfio"
                                 # PYTHIA > 8.200 may require library dl
EXTRAPATHS = ../lib /home/qdhan/MadGraph/MG5_aMC_v3_6_2/HEPTools/pythia8/lib l,-rpath,/home/qdhan/MadGraph/MG5_aMC_v3_6_2/HEPTools/pythia8/lib /home/qdhan/MadGraph/MG5_aMC_v3_6_2/HEPTools/zlib/lib l,-rpath,/home/qdhan/MadGraph/MG5_aMC_v3_6_2/HEPTools/zlib/lib # Path to the extra-libraries
                                 # Default: "../lib"
INCLUDEPATHS = # Path to header files needed by c++
                                 # Dir names separated by white spaces
ANALYSE = # User's analysis and histogramming
                                 # routines (please use .o as extension
                                 # and use spaces to separate files).
                                 # If the HwU.o files has to be linked,
                                 # it should be put *first*.