You hit on it exactly: it's the length of membership that's the issue. Though you have the posts for adult cat status, which generate the change in title, you don't actually have full adult status until you have 30 days membership, as well. Once you hit that milestone, the perks of adult status will kick in. (It may take a day for the system to catch up, so don't panic if it's not immediate.)
Okay, so it's built into the system then? I've not worked a whole lot with VBulletin from your side of things (as a member, yes, but never as a mod or admin) so I'm not sure what the differences are between VBulletin and other forum softwares, like IPB or phpBB
Thanks! It doesn't bother me that much, I was just curious!