Those specs are NOT "legal" limits. The DOT does not care what your owner's manual says.
The DOT has its own rules about what is legal and it's all about tire size, number of axles and spacing of the axles.
In the US, they do care about the GAWRs of the truck. In certain provinces in Canada, they care about the GAWRs and the GVWR of the truck according to the door jam sticker.
anyone know why the dual would be less than the single
As a tire roll, it generates heat, the more load, the more heat. This heat is dispersed by the air flow around the tire. With a dual, there is less air flowing around the tire so less cooling. To compensate, the load rating is reduced so less heat has to be dispersed.