Another explanation could be members that purchased premium and let their subscription expire. As far as I know, once you've taken advantage of some premium benefits, they won't go away unless you try to update them, like bigger size avatars and custom profiles.
The bigger avatar until you try change it no longer works but I'm not sure about customised profiles.
I have been searching and I found nothing, just custom avatars and user title questions. What is required to be able to color and customize your profile page?