'Author' in the WP context can refer to two different things: the person who has written a certain post/page or the role a user has.
Anyone who has sufficient rights on a site can be the author of a post/page. Admins and editors, for instance, do not have the role 'author', but they can publish posts.
The default user role author is in fact the 'lowest' role that gives a user authoring capabilities:
Note that the delete and edit rights in the case of the author role only extends to their own posts.