That's because in the immedient aftermath of the switch-over to the new site, they accidentally left in a spot where non-GDNet+ users could set custom avatars. While everyone was exploring features, some users such as myself figured out how to change our avatars thinking it was intended to be like that. They fixed it, but our avatars remained until the next time we attempted to change it (for me this was last week - now I have to use one of the stock avatars available to anyone). There are still a few non-GDNet+ users around who haven't tried to change theirs yet, and still have their custom ones, but it's only one or two users, so it's not a problem.

It adds value to GDNet+ by making yourself more noticeable to the community, and thus helps fund the site, so I don't mind the loss of my avatar too much. They have to reserve some things for the site's supporters.

moral of the story being not to change your avatar every month.