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 and when viewing user profiles. 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. If you go to the bottom of the page and change the drop down to Couples Blue the status is not shown.
What's the difference among Junior Member, Member, and Senior Member, on the profiles?
In vBulletin the admin can set profiles to default by certain parameters.
As of early May this MB had the profiles set by the parameter of number of posts and the threshold for "Member" set to 30, and "Senior Member" to 100 (Junior Member being the default otherwise).
However, it seems to be out of whack again (e.g. a few users with posts above 30 still show as "Junior" - marykay731, courtromo and a few with posts above 100 still show as "Member", as opposed to "Senior Member" - dianavr, randi&sherri).
The vbulletin manual states that an admin must "update user titles" after modifying user title parameters. Perhaps Randymon/Shandi/some_admin changed parameters and needs to do this still.