The problem with what you say is that the stomach is initially removed when you field dress the corpse, yielding 1 unit of human stomach. If the stomach is to be harvested, it should be from this point, in my opinion, because you’re removing the ‘guts’ of the creature.
THEN when you quarter it you get 4 pieces.
THEN when you quick-butcher the pieces, each piece yields an additional unit of stomach.
This means that from butchering 1 human corpse I get 5 units of stomach.

I again believe this is related to the code I listed above. It’s checking for a stomach each time you quick-butcher the quarter, as though you were quick-butchering a non-field dressed corpse. I don’t think this should be this way, I believe a stomach should only be checked when field-dressing or when butchering a full, non-field-dressed corpse.
Because of the way full butcher is coded, it does not grant additional stomachs when butchering the quarters.