I think you will find that for most light trucks the gross vehicle weight is less than the combined axle weight. GVWR is also a function of how well a vehicle will stop loaded up to that rating. GVWR is not caclulated by adding up the axle ratings or even tire ratings. Consider a dually, it has more than 12,000 pounds rear tire capacity, a rear axle that is around 9500 pounds and is on a vehicle with perhaps a 12,000-14,000 pound GVWR. Add in the front axle and two front tires and soon it is very apparent that if you try to make the numbers add up they won't.