Difference b/w geant4 and Fluka

Dear experts,
In my simulation, my CaSo4:Dy TLD is placed on an ISO slab Phantom having three Discs D1, D2, and D3 of CaSO4 under (1mm Cu+0.6 Al), Polystyrene and no filter respectively inside its cassette. While scoring doses on all these 3 discs, FLUKA and Geant4 results match for incident photons of energy of more than 50 keV but in lower energy geant4 has half dose deposition in D1 as compared to Fluka while dose opposition in D2 and D3 still matches for both codes (i.e. the is difference only for disc under Cu+Al filter).
G4Penvelope is used as a physics list and the range cut-off for both codes is taken as a 1 keV energy threshold. please see the attachments to help me find the reasons for the discrepancy.

physics.cc (1.9 KB)
RunAction.cc (6.4 KB)
SteppingAction.cc (5.0 KB)