The axle manufacturer does specify an axle limit which does not take into account the tires and rims on the vehicle.
The GAWR is the lowest of the tire and rims OR the axle manufacturer axle limit.
The front axle is selected to carry the weight of the truck with the heaviest engine and options. The springs are then selected to carry the particular combination weight of options selected at time of build. Some times, different axles are used in the front, but usually only two options.
The rear axle is usually selected to handle the torque of the engine. It is common for a manufacturer to use two different axles. Torque plays as much role as payload and why the axle limit exceeds the tire/rim limit (especially on SRW vehicles). Springs are selected to carry the maximum payload for the configuration.
Total GAWR always exceeds GVWR because it is almost impossible to distribute the weight evenly. Additionally, they manufacturers tend to leave a little margin to spare for design changes. This is also where brake design factors into the question. The brakes are designed to be able to stop the vehicle when it is loaded to GVWR within certain distance.
The other major factor is the frame design as it must be strong enough to handle the torque, payload, flexing, etc to keep everything together. Usually, the frame design does not impact the payload capacity other than the weight it eats from the GVWR.
Federal Laws also at one time had an impact GVWR. My 2004 Dodge as a lot of others had a GVWR of 9,900 pounds because that was the Federal Limit for commercial licenses at one time.
