It's a measure we're testing to impede spam.
The idea is that members have the 'probationary' status until they post for the first time, at which point they get 'new member' status. Probationary members will need mod/admin approval for their first post (once we implement the restriction). So the idea is just that a spammer will be caught before they have a chance to flood the board, and mods will only have to deal with a single post. For the time being, however, everyone needs to get sorted into the right membership categories. The system will update everyone once you've posted. So if you're currently looking probationary, but have posted, you'll get updated once you've posted again. Does that make sense?
It's not firm or anything yet, it's just a tactic to be tested.