I am discussing ammunition only because there is no life support supply consumption for GF in Aurora at all.
I'd like to have it, but...
Well, the supplies is all the equipment and support a unit need, ammunition is just a small part of it. Wealth take care of running cost for units that is not fighting and is an abstraction for replacing equipment over long times with no action. Remember that say 5t is the equivalent of 70 cubic meters of stuff for a single soldier, that is not just ammunition and the soldier itself, it is basically the size of a small apartment of stuff.
I know that Steve at one point said that he calculated the size of units and their supply needs based on that stuff, so supplies is basically everything a soldier or vehicle need to function and ammunition is just one part of that equation. So basing supply use around just guns and rate of fire seems odd to me, there are so many other things a unit consumes during combat. It is an abstraction.
You can't even assume that all units is actually firing any weapons in every eight hour increment, that is just an abstraction and a game mechanic. Conflicts does not really work like that. Supplies is everything from spare parts, fuel, food, power sources, transportation, field equipment, ammunition and more.