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Relationship between System CA, System Intensity and Flow (Total Out)

Hello,

I have a plot which is divided into sub areas, 10 to be specific, with individual outfalls. All outfalls are "Free Outfall". Each sub area is a gravity drained system served by catch basins and underground piping connecting to the respective outfall. All catch basins are subject to same rainfall intensity, ie. same time of concentration and same runoff coefficient. I am running this in GVF environment. 

after running the model i noticed that in some, 5 to be precise, of the outfalls, the product of "System CA" and "System Intensity" matches "Flow (Total Out)" but in other cases it does not. It can vary almost by 100%. 

I looked at a previous forum answer on flow balance at junctions in GVF, which does not seem to explain this situation because why would some outfalls, the product would match and not on the others. Secondly, the Tc at the outfall and the individual catchment areas is within a few percentage point, so intuitively the discharge rates should be similar. 

what could explain this difference?

thanks

KP