This is probably a dumb question, but why do the first class seats on the majority of U.S. domestic flights have less legroom than Y+ seats on BA/VS/BR?

For example, on UA/AA 2-class 767s, DL 767-400s, and nearly all other narrowbody first class seats, the leg room is usually 37-inch to 40-inch. On the other hand, British Airways, Virgin Atlantic, and Eva Airways, all have 38-inch economy-plus seats on their 747s from UK to Asia...