Could it be that to set up a "senior member" group you have to specify a post-count based level at which the group is automatically triggered, but allowing you discretion to enter people into the group manually? Could just be a setting that HAS to be entered as part of the VBulletin interface, I have no idea as I've never used VBulletin from the admin side.
The limit could've been set high, expecting only a few would reach it, so that it would still remain relatively up to the admins to decide. So that basically it IS up to the admins but in the odd case post-count beat them to it. Who knows? Well, Baz, as Fo said, so let's wait for him to answer this one