Why is this happening?
It is the rarest of the badges that you have. Not the rarest badge on the whole site. But since you only have one badge, it is also the commonest!
It would appear that the rarest badges on the whole site are
Lifeboat Answer score of 20 or more to a question score of -3 or less that goes on to receive a score of 3 or more (None yet awarded)
Publicist Share a link to a post later visited by 1000 unique IP addresses 1 awarded
Legendary Earn 200 daily reputation 150 times 1 awarded
Illuminator Edit and answer 500 questions (both actions within 12 hours, answer score > 0) 1 awarded
Tens of thousands of users have earned this badge. You just need to check the right links:
On meta, over 3000: https://ell.meta.stackexchange.com/help/badges/9/autobiographer
On the ELL main site, over 25000: https://ell.stackexchange.com/help/badges/9/autobiographer