I finally found an article that explains why the difference.
http://oppositelock.jalopnik.com/tow-me-down-1609112611
Short version. We go faster, therefor we need a higher tongue weight, therefor a lighter trailer.
Faster = less stable.
Higher percentage of the trailer weight on the tongue = more stable.
Higher tongue weight = less trailer weight for the same Maximum payload
http://oppositelock.jalopnik.com/tow-me-down-1609112611
Short version. We go faster, therefor we need a higher tongue weight, therefor a lighter trailer.
Faster = less stable.
Higher percentage of the trailer weight on the tongue = more stable.
Higher tongue weight = less trailer weight for the same Maximum payload