They wanted to leave things open, incase you wanted to make a freak or something I guess.
I agree. Thus the suggestion for a checkbox.
Also, the game isn't restricted to humans, we have Elves, and Beastfolk too. So those measurements wouldn't be accurate if you took them into account.
Ah, excellent point. They could provide different proportional guidelines of Bethesda's own design for the non-human options.