Logged in as admin, posts as someone else

I have a WP 2.6.1 install, works fine. I log in as admin, and go to post, and at the bottom for post authors I get two choices, the other two users in the blog, let’s call them john and mary.

When I post I have no option other than those two. So one or the other, but interestingly, not me. What’s going on? One of them is an Administrator, the other is an Editor, if that makes any difference.

Why won’t it let me post as me?

Richard