Well, Greek misspelling aside, those were two different names of the same man, Ramesses was his personal name and he wasn’t even royalty when he was born, his grandpa was vizier to pharaoh Horemheb back then, but Horemheb was old and and had no sons so he named the grandpa, later known as Ramesses I, as successor.
But since pharaohs weren’t simpletons like European monarchs, they needed more names, specifically four more, so the Ozymandias came not from Ramesses, but from his throne name: Usermaatre Setepenre.