According to wikipedia, Rameses II had an "aquiline nose and strong jaw line". Pictures of his mummy also show a fairly wide mouth. But contemporary statues of him show a much different face. Was this artistic liscence? Poor art-manship (hard to believe)?
Egyptian art is idealized, not realistic. Artists were concerned with creating a sense of strength, perfection, and timelessness, not crafting accurate portraits.
The process of mummification and the intervening years change the soft tissue and sometimes even the bone structure that defines a persons features.
Mostly it's because he didn't look like a mummy when he was alive.