Receiving badge before the requirements are met

Question

Just a few hours back I received the nice answer badge for this answer of mine which I posted few months back.

The nice answer page says that (to receive the badge) :

Answer score of 10 or more. This badge can be awarded multiple times.

But I currently (at the time of posting this question) have only 9 upvotes on the answer :

I initially thought this might be because of 10 upvotes and a downvote. But the reputation history contradicts that assumption as there are no downvotes on my posts.

So my question(s) are how did I receive the badge even before the requirements are met? are there other factors (other than the votes) that determine score of an answer? Is this a bug?

Answer 1

Upvotes can be reversed, under certain conditions. Most likely your post reached 10 score at some point, and then one of the upvoters undid an upvote, bringing it back to 9. The badge must have been awarded while you still had 10 score. Once awarded, badges remain forever.

This is most likely what happened, and there is no bug.

As you were awarded the badge a few hours ago, and right now you have 9 upvotes and no downvotes, this upvote + unupvote must have happened today. Note that when an upvote is reversed quickly like this, it won't show up in your reputation history. If the upvote reversal happens on the same day, I think the upvote + unupvote is removed from the history. If the day changes in between the two actions, then it will be visible as a +10 and a -10.