I'm going with non-canon.

The production team, I heard, chose the Griever form cause they wanted Ultemacia's EX form to stand out and yet be instantly identifiable for the fans. Its modified cause they probably couldn't get her real version to work in the game specs, both EX-Death and Sephiroth were also modified cause their true ultimate forms couldn't work with the games specs. Thus they were toned down or completely modified. Though I hear Nomura enjoyed making Ultemacia's new EX form.