What @KKC said: fitted sheets have to have the same dimensions as the mattress. With a comforter/duvet/duvet cover there is some wiggling room. On a full-size bed, the full/queen size comforter/duvet/duvet cover will just hang a bit lower than on a queen-size mattress.
I guess the six-inch differece in mattress width and five-inch difference in length is sufficiently insignificant to allow one to use the same size of cover, while being too large to allow for a one-size-fits-all fitted sheet.
Hope this helps decrease confusion on this topic.