You'll get all kinds of wild and interesting/head scratching opinions and theories on this as its another topic that gets beat to death. Only the vehicle mfg can tell you why he chose a GVWR or the GAWRs for a particular truck.
Some of the newer gen trucks with those high GVWRs are within 200-300 lbs of the sum of the GAWRs in some cases depending on the trucks configuration. For that reason some owners have complained of going over their trucks RAWR from using GVWR to determine a payload. Folks forget a GVWR payload is over the trucks GAWRs and not just setting the rear axle/tires.
The vehicle mfg sets the trucks GVWR and the GAWRs. The GVWR cannot be more than the sum of the GAWRs ratings. This is one reason GVWR out here in the real world does not legally determine gross weight or a payload on a truck in the states or any province.