How does a -1 vote happen?

I'd asume someone downvoted, then removed the downvote. The original downvote didn't get through, but the reversal did. Something like that

I have -1 communities in my instance because I made a test one and deleted it early on.

A fediverse specific glitch.

Someone put in the vote?

My guess is that the dislike was removed.

I think from something I observed a couple of weeks ago, someone (or a couple people) downvoted and then their account was deleted or banned.