A lot of those numbers are driven by federal regulations on truck "class" designations. In order to be considered a 3/4 ton truck, the truck can't gross more than like 8800 or 9k pounds. Based on those numbers, you might have a 3/4 ton diesel truck that only has a payload capacity of 1200 pounds. But based on the axle ratings on the door sticker, you might be able to gross up to 11,500. NC has a provision to allow a truck to carry more weight if it's been modified to do so. The axles and brakes are the same as the DRW truck of the same year, so add some aftermarket tires and airbags, and now you're good for maybe 12,500 gross. Then hook a trailer to it, etc.
The "towing capacity" numbers used to be generated by the manufacturers. It was a marketing number, regulated to some extent by their attorneys. Nowadays, everybody's supposed to abide by an SAE standard. It's driven by performance capabilities of a stock truck, though.
