can't get the gvwr math to work

I'm not trying to argue for or against. Everyone has a rationale for the decisions they make, payload weights being no exception. Simply stating a fact. Either you go by the manufacturers' ratings or you decide to go by a rationale of your choosing.

Manufacturers' ratings can depend on any factor on the truck, so although folks focus on what they think are the obvious ones, it does not mean that is what the rating is based on.