Weight rating, 5er vs conventional

I may have missed it reading through the posts but I think the major reason for the difference between 5th and ball type trailer ratings is the hitch on the vehicle. A ball hitch comes in different classes 1-5. A vehicle with a class 5 hitch (which they really only recently started putting on trucks previously class 4) have ratings of max weight and max tongue weight. IE a class 4 hitch has a max weight rating of 10k weight and 1k tongue weight same hitch with weight distributing hitch is 14k and 1400 lb tongue weight. A class 5 hitch ups it to 12k, 1200 tongue and weight distributing hitch up to 17k or 1700lbs tongue weight. As you can see the tongue weight in the ratings is at 10%. Most trailers at 10% hitch weight are going to be hitch light. You move that up to 15% and that 17000lb rating becomes more like a hitch weight of 2550lbs well over the hitch tongue rating. To keep the hitch weight at 15% at the rating of 1700lbs your trailer can only be 11333 lbs(even a 12% tongue to trailer weight ratio is well over the tongue weight rating of 1700lbs at 2040lbs). Where as with a 5th hitch your limiting factor is not really the hitch but by payload rating of truck, rawr or tire rating for the truck depending on what you prefer to go by not the hitch. I have towed some heavy trailers over the years and I personally would not tow a 17000 lb trailer with a ball hitch with a srw truck. Even a dually would be questionable for me too much tail wagging the dog by that point. Not saying people don't or cant but not for me.