(whichever is smaller)

I'm contemplating that you uploaded your 100x100 image before you were UG Bronze. The admins probably set the permissions for regular members to be 100x100 and forgot to set it for UG. When you got to UG, your max dimensions dropped to 50x50, but it didn't mess with you avatar since it was already set before you got to UG. Makes sense from a programmer's perspective. And from a what-an-admin-is-likely-to-do perspective.