Not all trucks are equal. A regular cab 4x2 truck will inherently have a higher payload capacity than a heavier 4x4 Screw truck will have as these trucks are currently built on the same chassis.

The 2012 Scew trucks will have some frame re-enforcements enhancements that will enable it to carry a similar payload as the reg cab trucks.

It makes sense from a business standpoint to offer this as these trucks fetch far bigger bucks than a reg cab 4x2 XL does.