I think I can give a more detailed explanation than abear (I think you were on hiatus when this happened).
Like abear said, early members (even before me) could use custom titles (IE abear having "ectomorph"). Everyone else who didn't change the custom title, was saddled with junior member. It changed either with time or postcount although we never really determined which.
When I joined up, everyone got registered user, period. No matter how many posts you have or the time you've been here. One day however, I logged in as a registered user, and something was wrong. I (and others) could see forums we weren't supposed to be seeing, and when we posted in those forums our user titles changed from registered user to senior member. Some glitch turned on the other titles for a few other users.
I don't think anyone with registered user will ever change that title unless an admin does it for you. Now I could be wrong, but I'm pretty sure how it works.