In SAO season 1 Akihiko Kayaba reveals that he is the final boss, but in the movie 'Ordinal Scale' the final boss is something else. Why is this?
According to an Interview with Kawahara (original author of SAO), abec (SAO's illustrator), Miki (SAO's editor) and Itou (Ordinal Scale's director), the final boss shown in the movie, "An Incarnation of the Radius" was the original boss of the 100th floor designed by Argus, but Kayaba found it so broken, he decided he'd take its place as the final boss.
Quoting certain parts of the Interview:
Kawahara: The last boss that was present in the 《Ruby Palace》 was the one formally developed by 《Argus》 before Kayaba took over.
and later on:
Kawahara: The reason why Kayaba changed himself to the last boss was because he saw the data for the original boss and thought "Like hell anyone can beat this thing" …or at least that's what I think he did.
Source: translation of said Interview