I am the admin and I have two other users that are classified as contributors: Contributor A and Contributor B. When I, as admin, go to write a post/page the author defaults to Contributor B and my only other option is to choose Contributor A. There is no option to choose the Admin as the author.
In posts that I'd written prior to bringing on Contributor B, admin was an option.
Also, all posts and pages written by the admin are still showing as such.