Can anybody who can explain how and why this happened. Just really curious about this case. Thanks.
Upgrade00 already explained this.
The Avatar was worn when he was a full member. That time there was no merit and ranks were determined only based on activity. Some posts were deleted, the activity decreased to less than 100 and he became a member.
When the merit system kicked-off, all users with member rank including this user received 10 airdropped merit.
So, there is no bug in ranking or merit system.
The bug is that this user is still wearing the avatar worn when he was a full member although he shouldn't be able to wear avatar.
There's a same issue for some old users that are below full member but still wearing avatar (In the past all users even newbies were able to wear avatar).
Examples:
NinazudvdrewritableDrKenobivelona2589rate5