The f350/f450 chassis cabs have the same axle ratings:
Front 7500 lbs
Rear 14706 lbs
The f350 dually/f450 4x4 pickups axle ratings:
Front F350 4x4 5600 lbs
F450 4x4 6000
Rear 9900 lbs
The f350 4x2 has a rear axle rating of 10300 lbs
The 4x2 f450 has a rear axle rating of 9900 lbs
The f350 4x4 SRW axle rating
Front 5600 lbs
Rear 7230 lbs
The f250 4x4 axle rating
Front 5600 lbs
Rear 6340 lbs
So it makes sense that the lighter f350 drw pickup has a heavier payload than the f450 pick up.
The fact that the GVWR of the f450 pickup is 2500 lbs less than that of the f450 chassis cab seems to have some merit. The f250 no longer has the same axle rating as the f350 SRW.