genres are used to provide a classification for a movie, so in my mind they are fluid.

For example.

if you are trying to decide what part of a product catalog to list it in, you will be using one set of criteria, and will probably list it as Sci Fi

if you are talking to a friend and comparing Star Wars to any other sci-fi movie, you are then more likely to reach for more descriptive, specific genres, and will say that Star wars is more of a Space Fantasy than your typical sci-fi story.