I'm no expert, but I can speculate on the Internet with the best of them...
Beyond following laws and satisfying insurance, the rest comes down to physics and economics. Manufacturers set tow ratings and payload ratings based on longevity and capability testing or how their parts have been spec'd. A certain engine, driveline, and chassis can manage to achieve a certain longevity and performance capability with a load when it fits into their specs. Can you haul or pull more than the rating? Sure, but your vehicle might wear out faster or fail faster. Manufacturers want to avoid warranty claims and preserve quality reputation so they ensure that how they rate vehicles will generally meet those goals.
Changing axle ratios changes the amount of torque required to propel the vehicle. Torque relates to stress on parts and wear. Vehicle plus a payload has a mass that takes a particular force to move it and stop it and that force comes from engine power transferred through the transmission and converted to force on the ground with torque through the axle and tires. Torque and stress on components changes when you do things like change an axle ratio. A particular engine and transmission can handle moving a load for the vehicle expected life with one ratio but if you change the ratio it might cause failures earlier. When it comes to automatic vs. manual transmissions then you have to consider things like power losses due to the fluid dynamics in an auto trans, weight of the transmissions, cooling factors placed on the vehicle's cooling system, etc. Every change in a component changes the total vehicle performance in some way and the manufacturer has taken that into account in their formulas. As I said above, all of this is independent of the legalities and liabilities that restrict things. But manufacturers make those assessments and then decide on the ratings. I suspect they are not above changing the ratings to best their competitors even if no actual component changes happen, up until their warranty claim costs rise or quality reputation falls enough to make it not worthwhile as compared to sales.
One thing that is fairly consistent is that manufacturers and laws always play it safe. Exceeding any prescribed limits by a small amount will probably never cause an immediate disaster. The further you get from the prescribed limits the more likelihood for problems. So pulling 11,500# with a truck rated for 10,200 once in a while on good roads is, in my opinion, OK to do. Pulling 20,000# probably not so much.
Rob
