identifying the source for a change in the cross section - S3 leptoquark and SM

Dear MadGraph experts,
I'm trying to understand the source for a change in a cross-section I'm calculating - whether the change comes from an interference? or something else.
I consider the process 'p p > mu+ mu-' with up to 2 jets in the 5 flavor scheme (5fs). One time when only SM is involved, and a second time when there is a leptoquark S3 particle in addition to the SM.
I'm using a S3 leptoquark model, 'LO_LQ_S3', that can be downloaded from 'https://lqnlo.hepforge.org/downloads/?f=LQ_NLO-0.0.1.zip'.
I've created two restriction files: 'restrict_no_b_mass.dat' and 'restrict_no_b_mass_LQ_YUK_OFF.dat', which content is listed below. In the 'restrict_no_b_mass.dat' file I put the b mass and the b yukawa term with the higgs to be zero (5fs), and set the yukawa terms of the leptoquark with the b and \mu and with the s and \mu to be 0.01. In the 'restrict_no_b_mass_LQ_YUK_OFF.dat' I put all of the above parameters to be zero - essentialy I set the 5fs and decouple the LQ from the SM.
I then use the scripts which are listed below to calculate the cross-section in each case. The results I get are:
1. no_b_mass_LQ_YUK_OFF
   -----------------------------------------
 Cross-section (pb) Events Data
 ============== ===== ====
  1128 +/- 1.2 50000 parton
 1043 +/- 1.8 46237 pythia8

2. no_b_mass
   ------------------
 Cross-section (pb) Events Data
 ============== ===== ====
 1107 +/- 1.3 50000 parton
 1019 +/- 1.8 46010 pythia8

So, obviously there is a change in the cross-section.
Next, I wanted to understand the source for this difference. Hence, I entered the results.html page and went to the table of the leading process 'P0_qq_ll'. This is what I see:
1. no_b_mass_LQ_YUK_OFF
   -----------------------------------------
Graph Cross-Section Error Events (K) Unwgt Luminosity
G2 726.3 0.798 551.474 44246.0 60.9
G1 249.1 0.686 178.74 21987.0 88.3

2. no_b_mass
   ------------------
Graph Cross-Section Error Events (K) Unwgt Luminosity
G2 725.4 0.787 558.72 44925.0 62
G1 220.5 0.802 116.76 14618.0 66.3
G4 9.754e-08 4.53e-09 7.01 29.0 2.97e+08
G3 7.56e-09 2.96e-10 7.038 118.0 1.56e+10

Then, I followed 'https://answers.launchpad.net/mg5amcnlo/+faq/2944' to identify the diagrams that contribute to each integration channel.
Suprisingly I've found out that in both cases the G2 integration channel gets contributions from the Z propogator *only*, and the G1 from the photon propogator *only*.
Thus - why do we see a change in the G1 integration channel cross-section? does it come from an interference with the LQ diagrams? and if so, why are these diagrams not included in the G1 integration channel as well (the diagrams that include a LQ appear only in the G4 and G3 integration channels)? if it's not interference then what is the cause for this change?

Thank you,
Ophir

======================================================================================
restrict_no_b_mass.dat content:
======================================================================================

###################################
## INFORMATION FOR SMINPUTS
###################################
Block SMINPUTS
    1 1.279000e+02 # aEWM1
    2 1.166370e-05 # Gf
    3 1.184000e-01 # aS

###################################
## INFORMATION FOR MASS
###################################
Block MASS
    5 0.000000e+00 # MB
    6 1.720000e+02 # MT
   15 1.777000e+00 # MTA
   23 9.118760e+01 # MZ
   25 1.250000e+02 # MH

###################################
## INFORMATION FOR DECAY
###################################
DECAY 6 1.50833649E+00
DECAY 23 2.4952e+00
DECAY 24 2.085e+00
DECAY 25 4.07e-03

###################################
## INFORMATION FOR YUKAWA
###################################
Block YUKAWA
    5 0.000000e+00 # ymb
    6 1.720000e+02 # ymt
   15 1.777000e+00 # ymtau

###################################
## INFORMATION FOR LQPARAM
###################################
Block lqparam
    1 3.000000e+02 # MS3

###################################
## INFORMATION FOR YUKS3LL
###################################
Block yuks3ll
    1 1 0.000000e+00 # yLL1x1
    1 2 0.000000e+00 # yLL1x2
    1 3 0.000000e+00 # yLL1x3
    2 1 0.000000e+00 # yLL2x1
    2 2 1.000000e-02 # yLL2x2
    2 3 0.000000e+00 # yLL2x3
    3 1 0.000000e+00 # yLL3x1
    3 2 2.000000e-02 # yLL3x2
    3 3 0.000000e+00 # yLL3x3

======================================================================================
restrict_no_b_mass_LQ_YUK_OFF.dat content:
======================================================================================

###################################
## INFORMATION FOR SMINPUTS
###################################
Block SMINPUTS
    1 1.279000e+02 # aEWM1
    2 1.166370e-05 # Gf
    3 1.184000e-01 # aS

###################################
## INFORMATION FOR MASS
###################################
Block MASS
    5 0.000000e+00 # MB
    6 1.720000e+02 # MT
   15 1.777000e+00 # MTA
   23 9.118760e+01 # MZ
   25 1.250000e+02 # MH

###################################
## INFORMATION FOR DECAY
###################################
DECAY 6 1.50833649E+00
DECAY 23 2.4952e+00
DECAY 24 2.085e+00
DECAY 25 4.07e-03

###################################
## INFORMATION FOR YUKAWA
###################################
Block YUKAWA
    5 0.000000e+00 # ymb
    6 1.720000e+02 # ymt
   15 1.777000e+00 # ymtau

###################################
## INFORMATION FOR LQPARAM
###################################
Block lqparam
    1 3.000000e+02 # MS3

###################################
## INFORMATION FOR YUKS3LL
###################################
Block yuks3ll
    1 1 0.000000e+00 # yLL1x1
    1 2 0.000000e+00 # yLL1x2
    1 3 0.000000e+00 # yLL1x3
    2 1 0.000000e+00 # yLL2x1
    2 2 0.000000e+00 # yLL2x2
    2 3 0.000000e+00 # yLL2x3
    3 1 0.000000e+00 # yLL3x1
    3 2 0.000000e+00 # yLL3x2
    3 3 0.000000e+00 # yLL3x3

======================================================================================
The script which is using 'no_b_mass' content:
======================================================================================

import model LO_LQ_S3-no_b_mass
define p = g u c d s u~ c~ d~ s~ b b~
define j = g u c d s u~ c~ d~ s~ b b~
generate p p > mu+ mu- QED<=99 @0
add process p p > mu+ mu- j QED<=99 @1
add process p p > mu+ mu- j j QED<=99 @2
output LQ_YUK_bmu_smu_pp2mumu

launch

shower=Pythia8
analysis=off

set pdlabel lhapdf
set lhaid 263000
set maxjetflavor 5
set asrwgtflavor 5
set nevents=50000
set ebeam1 6500
set ebeam2 6500
set use_syst False
set lqparam ms3 300
set yuks3ll yll2x2 0.01
set yuks3ll yll3x2 0.01
set ickkw 1
set xqcut 50
set ptj 0
set ptb 0
set drjj 0
set drbj 0
set drbb 0

done

======================================================================================
The script which is using 'no_b_mass_LQ_YUK_OFF' content:
======================================================================================

import model LO_LQ_S3-no_b_mass_LQ_YUK_OFF
define p = g u c d s u~ c~ d~ s~ b b~
define j = g u c d s u~ c~ d~ s~ b b~
generate p p > mu+ mu- @0
add process p p > mu+ mu- j @1
add process p p > mu+ mu- j j @2
output LQ_NO_YUK_pp2mumu

launch

shower=Pythia8
analysis=off

set pdlabel lhapdf
set lhaid 263000
set maxjetflavor 5
set asrwgtflavor 5
set nevents=50000
set ebeam1 6500
set ebeam2 6500
set use_syst False
set ickkw 1
set xqcut 50
set ptj 0
set ptb 0
set drjj 0
set drbj 0
set drbb 0

done