diphoton mass cuts in run card not working?

I'm trying to generate pp -> bb+diphoton events in MG5 with:

define p = g u c d s u~ c~ d~ s~
define bs = b b~
generate p p > a a bs bs

In my run card, the relevant cuts for photons are set to:
 30 = pta ! minimum pt for the photons
 -1 = ptamax ! maximum pt for the photons
  0 = ea ! minimum E for the photons
 -1 = eamax ! maximum E for the photons
 3 = etaa ! max rap for the photons
   0 = etaamin ! min rap for the photons
 0.4 = draa ! min distance between gammas
 0.4 = draj ! min distance between gamma and jet
 0 = drab ! min distance between gamma and b
 0.4 = dral ! min distance between gamma and lepton
-1 = draamax ! max distance between gammas
 -1 = drajmax ! max distance between gamma and jet
 -1 = drabmax ! max distance between gamma and b
 -1 = dralmax ! maxdistance between gamma and lepton
 90 = mmaa ! min invariant mass of gamma gamma pair
 170 = mmaamax ! max invariant mass of gamma gamma pair

It doesn't look like I can post plots here, but what I see is that 5-10% of the time, the diphoton mass is not in the 90-170 GeV window defined above. It can often, in fact, but much, much lower (all the down to ~10 GeV, even). Is there a reason for this? I see the same problem in diphoton + b + jet as well as in diphoton + dijet production.

Many thanks for any advice and/or help. I'm using MG5_aMC_v2.0.2, and running mg5. I can post full run cards, etc., if that would be helpful.

Best,
Jahred