The accounts may be created, but not approved... so could they log in but not post anything (until approved)? So, you see non-approved users in members online, but they're restricted... it must be that non-approved users are set to not be able to Post threads / Post replies? They also shouldn't be able view the online member list?
I can't remember from when I signed up... is email verification enabled? It should be the first hoop for a new user to jump through. EDIT: If email verification is enabled, and not completed by the user, then maybe they don't hit the approval queue yet?