I believe the general idea is that the GVWR on 3/4 to 1 ton trucks is set by the manufacturer based on where they want it to fall in various classes.
All 2500's, IIRC, max out at 10K (some older ones may be lower). Puts them in s specific weight class even if all the equipment is the same as the respective 3500. In a lot of years, the only difference was that the 3500 version had extra rear overload springs.
This is not 100% always the case, but it explains why the totaled GAW doesn't equal the GVWR.
So basically, I guess I am agreeing with your take on it.
If I'm wrong, I'm sure someone will let us know! :)