Not sure why they have that rating but I can tell you I have been at or near the 3k mark in my bed and the truck does fine. It squats a little but that's expected.
I know this is not exactly the same but here is quite a few retaining wall caps on a pallet plus a dump trailer hooked up with a decent tongue weight and you can see the sag. Usually the rear is about 1-2" taller than the front with nothing hooked up or in the bed.
