Really, 3/4 tons are not underrated. The difference is, they are rated for continuous use. You can load one to GVWR on day one and drive it 100,000+ miles with only normal expected wear and tear.
Try and do the same thing with a 1/2 ton. It will be ready to fall on a pile at 100,000 miles. They are only rated for occasional use at their maximum capacity.
Manufacturers don't state this explicitly on paper because they don't have to. They want you to think your 1/2 ton truck is some mean hauling beast, and it is, all the way home from the big box lumber store. Now take that pallet of Sakrete from Boston to Chicago and you'll see where the differences lie.