What’s going on there is that there is a delete vote registered against something you are not allowed to delete because it is no longer negative, and so it does not show up as having any delete votes accounted to it since there is no delete button to show the number in parens after. But it is still there, and it is still meaningful — you just cannot normally see it.
What you can do is give it a temporary downvote and then reload to see the actual delete count:
Then you can renege on the delete vote if you want, and bring it back to a net zero votes.
Things can get this way in one of two ways. It may have been upvoted since the delete vote was cast. Or, as is more likely, a 20k user cast a delete vote on a zero-vote answer from the Low Quality queue, where that is explicitly permitted.
If you go through the Delete queue form the 10k tools page, you will seem some answers showing up on the recent delete votes listing, but which are greyed out because you cannot cast a delete vote. Both situations can give rise to this situation, but in my experience, it is the delete-vote-from-review-queue scenario which is the more common of the two.
There are other reasons for things being greyed out in the 10k tools, too. However, in my opinion, more could be done in that area to save time. The most important is that if you have already close-voted a question or delete-voted either a question or an answer, it would really help save time if that post were greyed out from the 10k tools. But it never is. Yet.