Single tires hold more weight than duals?

More inflation, or it can shed heat faster a s a single.

Dual, as in 2 tires on one side of one axle. Not tandem axles.

It's because the tires can easily be subjected to uneven loads when on uneven terrain. There's no equivalent of the equalizer which tandem axle setups have - the tires and wheels are hard coupled together. In an extreme example, you can picture driving over a curb at an angle, where one of the tires would take almost all the load for that side.

duals can't shed heat as fast for one thing.

May have something to do with heat generation and heat dissipation. Still though, 2778 x 4 is a lot more than 3042 x 2, and will likely be well above the RAWR on most of our trucks. My RAWR is 7400, but my tires are rated for a combined 11,112. that's a lot of cushion.

space between tires is pretty critical. The inside tire puts more stress on the axle. (IIRC)

A lot of folks understate the necessity of absolute identical matching of paired dual tires. Manufacturer, model, tread depth, and of course, pressure.

One day when you have a bit of time, work out the geometry of differential radius from axle centerline to end. What is the percentage of "scrub"? Oooooo it's not only distance but being locked in tandem.

OP here...Thanks for replies

Think I finally found a reason, at least in my mind

After lots of looking around found this

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=71

"Additional considerations require that all four tires on the rear drive axle be equivalent. Ideally this means tires should be the same model and have identical remaining tread depths. Any discrepancies between the four tires will result in the taller tire(s) being forced to carry more than their fair share of the load.
Light truck tires that are intended to be used in dual applications have two "Max Load" ratings branded on the tire's sidewall. This is because the load capacity rating of a tire serving duty in a "single" application is greater than the exact same tire being used in a "dual" application. For example, a LT235/85R16 Load Range E tire is rated to carry 3,042 pounds when inflated to 80 psi and used in a single application, but only 2,778 pounds when used in a dual tire application at the same inflation pressure.
This reduction in rated load capacity results in slightly larger tires being specified for the application that can better withstand...

the additional stresses experienced when a single tire goes flat and the three properly inflated remaining tires are required to temporarily carry the load at reduced speeds to remove the vehicle from immediate danger. This load reduction is not a concern for vehicles originally equipped with dual rear tires because the vehicle manufacturer factored it in when they specified the tire size for the vehicle."