Today I reviewed some posts in the Low Quality Posts, and I hit the daily maximum. But at the "Today" field, it shows 19, but I got the message:
Thank you for reviewing 20 low quality posts today; come back in 14 hours to continue reviewing.
Here is a screenshot:
When I manually counted my reviews of today, I counted 19 reviews. The reason can't be that one of the posts that I've reviewed is deleted, because I reviewed 10 posts that are deleted now.