"Unread" - not working consistently

[hungryonion]

"Unread" numbers incorrect

I pretty frequently see a number of posts next to the Unread button, but most of the time when I click it there are no unread posts. Other times, it will say, for example 2 Unread, but when I push the button five or six will show up. This doesn’t seem to happen on the other board that uses this software.

Which threads are causing the issues?