Alright, I did a bit of digging/asking... and I don't know that I can present a clearer answer.
As suspected, there is far more that goes into advertising a payload than just a simple NHTSA standard, which makes it
very difficult to compare manufacture to manufacture or even across different models from the same manufacture that have different engineering/legal teams. Those factors include NVH aka ride quality, fuel economy, mechanical capacity, suspension capacity, wheels, tires, brakes, bearings, diffs, CAFE standards and absolutely legal ramifications of customers exceeding payload/towing, all of which goes into their decision to land at any given number. An example that was offered is the same vehicle having different payloads in different markets, not because the testing is different but rather market emissions requirements could change based on GVW and if it's considered a cargo vehicle versus a passenger vehicle. So they may sandbag in market x to land in an emissions bracket that better suites their overall fleet standard (see CAFE). On the other hand a competitor, Company X, could test/certify their economy and payload on a package that they don't really even manufacture other than on paper, i.e. you'll never find it available on a dealers lot. Pretty fascinating and confusing really, it sounds like it's a moving target too.
Additionally, if a certain package or trim makes up 33%? or more of their total volume, NHTSA says they have to test that too so that could introduce why some trims are kept at a lower mix rate. Even though said trim could have a better payload due to better suspension, it could also hit them on the CAFE side so they only make 25% of their total volume in those packages. In Australia (and other markets), it's very popular for aftermarket companies to offer GVW upgrades which is generally comprised of better suspension and in some cases brakes. But often the axles, frame, steering, etc are completely un-touched. Some of those GVW upgrades are simply suspension and now it legally has a higher legal payload. Manufactures won't do that as it could both hurt their NVH factors that
most consumers want AND bump them into a different emissions standard, import tax bracket, etc. Then some include axles in their GVW upgrade kits and make you pull the rear seating so it's now a "Medium Goods Carrying Vehicle" instead of a passenger car. I'm glad we're not there yet in the US