"They" (whoever you want to blame for the tongue/pin weight percentages) all started with the way SAE 2807 defined the tongue weight standards.
In paragraph 4.4.2.4, SAE defines the process to adjust the ballast on conventional trailers for the tests. It instructs to adjust the ballast to attain a 10% tongue weight based on the total weight of the trailer.
In paragraph 4.4.2.5, SAE defines the process for fifth wheel and gooseneck trailers the same way with the ballast adjusted to attain a total pin weight of 15% of the total trailer weight.
So, since "they" defined and conducted the tests using 10% and 15% as the weights, "they" advertise them in their charts... It matters not that NONE of us will ever see that kind of tongue/pin weight on a travel trailer, "we" are a small, (likely insignificant percentage) of trailer users.... I'd suspect that most boats, utility trailers, gravel trailers can (and are) capable of attaining tongue/pin weights that reflect 10% or 15% of their total weight... For us, with travel trailers, it's a fantasy to even believe it's attainable, much less that the majority of us will ever see such hitch weights in practice.....
You can download the entire SAE 2807 test procedure and doublecheck the paragraphs above to get you to the "section pertaining to trailer weights" at this link:
https://www.52yufa.com/download?id=58 NOTE: There are two advertisements on the link, one above and one below the actual download tab. Don't click on the advertisements, click on the blue "download" tab located between them.