Maybe he went to Full Member and saw a lot of his posts deleted and was able to keep the avatar ?
That is also a possibility. If you look into their post history, you can see a fair amount of posts are just '.' (second page). This means that the user has likely edited their posts. Those eventually get reported and deleted making their activity drop (unless they create new posts).
Wel,, I think we found the reason for that strangeness. So you confirm that if some Full Member lose its rank, he will keep its avatar ?