It seems as though vehicles intended for the consumer market are that way. This is my theory, and it's just a theory, I have nothing really to back it up with- Say your truck's GVWR is actually the sum of FGAWR and RGAWR, so 8850. Say your truck weighs 5500 empty, giving you a max payload of 3350. The average consumer might see that number and assume they can just throw 3000 pounds in the bed. The problem with that is the RGAWR would be exceeded substantially. Therefore the OEMs give some "wiggle room" with GVWRs and payload ratings to help avoid exceeding axle ratings by a wide margin.
The GVWR on trucks for the commercial market typically is FGAWR+RGAWR. My theory on that is a commercial user is more likely to pay attention to axle weights, and the OEMs leave it to the truck user to determine how to load the truck.