It is "decided" by the number of posts a user has made. This is only viewable using the VB4 Default skin and not the Couples Blue skin. This caused a lot of confusion among users when Couples upgraded to the vBulletin message board software back in 2009 and was hence removed from the skin at that time.
I believe you are recalling correctly, however a grab of sample data shows that doesn't appear to be what is going on here.
The vbulletin manual states that admin must "update user titles" after modifying user title parameters.
Perhaps randymon/couples_admin/someone needs to do that (can't imagine it being very important though).
note the inconsistency of titles in this list sorted by # posts:
(uname, #posts, UTitle)