I think I've already mentioned this in this thread, but there is an alternative conclusion that you can draw from the data of comparing ROW to NA vehicles. For two identical vehicles, one NA and one ROW, to have different ratings in terms of capacity for weight, there are only two options:
1) The ROW rigs are being pushed into the safety factor beyond what NA rigs are as you suggest.
2) The NA rigs are under-rated and actually have higher capability than the ratings claim.
For 1), it seems reasonable, but when it comes to regulations, Australia makes California look positively libertarian. From what I read, Australia actually does fine people and impound vehicles if they are overweight, in addition to their other regulatory layers -- bullbars have to preserve airbag function, tires cannot stick out past fenders and are limited in overall size, etc. Caveat - I've never lived there, but this is what I read.
For 2), it is a bit interesting and I dare say suspicious that our mid-size trucks all stay within the 6000 lbs class as required by CAFE standards, almost irrespective of their componentry or ROW equivalents. Manufacturers need to hit a certain emissions target, based on the number of vehicles in a given class that they sell. So by ensuring our mid-size trucks stay in a given class, they are supporting their overall fleet emissions targets. If they went to a higher class, while payload and other things might increase, the emissions targets would not be met, thus resulting in them not being able to sell vehicles.
Candidly I have no idea which answer is the correct one. But, given that folks in Australia have been using "mid size" trucks in the trades and industry for decades, it suggests that perhaps option 2) above is more likely than option 1); if option 1) were to be true, I would expect that vehicle issues in Australia related to weight (so, safety and reliability) would vastly outstrip the same metrics in North America, but that does not appear to be the case, with 4.5 deaths per 100k in Australia as compared to 12.5 deaths per 100k in the USA. This is not a definitive statistic of course and there are other factors that contribute to these stats (like moose vs kangaroo strikes!) and there are likley other factors influencing the fatality rate, but I'd still expect Australia to have more accidents than they do if everyone is driving around an unsafe rig all the time, but in fact they are 21 in the world for least amount of accidents.
An additional perspective is that only recently have "Full sized" trucks made inroads in Australia, largely for the recreational market; mid-size trucks in NA are just "trucks" for most of the rest of the world, so going back to what I mentioned at the start of the last paragraph, they use these "mid-size" trucks extensively in trade and industry in ROW applications. If they were breaking down all the time or failing, I suspect that the HD-level trucks would be just as popular in Australia as they are here. You can get them, but they are not very popular, because the mid-sized trucks (with their much higher GVM ratings as compared to NA trucks) seem to get the job done for them just fine.
All of these perspectives together suggests that there is no obvious technical reason for a NA Colorado to have a 1,000 lbs payload gap as compared to the ROW Colorado, which further underscores that GVWR in North America seem to primarily be a result of regulatory frameworks, not an engineering/physics decision.
But it's all speculation - as
@DRAX said, manufacturers don't provide us their info so only the lawyers (And probably engineers!) know for sure. So, I do what I can to enhance my truck's ability to carry it's load, but when loaded I still try to stay within design parameters from the factory. But, I also don't stress these days about being +/- 10% payload in my rig (a couple hundred pounds).
