Correct me if I am wrong, but here is my hypothesis from my experiences:
You can review 20 posts per review queue. However, you have 24 close votes a day. The system took your vote because it was not in the review queue for the day, but it still counts towards your 24 close vote per day limit. In the review queue, both the close and not close options count as a review (skips don't), but only those that you actually give a vote to close response to actually count toward your close votes per day.
I agree that the fact that the review queue entry page says "20 Close Votes" and that is misleading, but that is perhaps because "Close Votes" is the name of the review queue. This should be made more clear that it is in fact the review queue and not close votes overall.