On occasion, the green post count box includes deleted posts in the number visible to ordinary (non-mod) members, resulting in some weird numbers, such as 7 of 5. It clears on navigating away from the thread and returning.
So far, I’ve only spotted this happening on Chromium, but that may be a red herring.
(@zogstrip says this is a known issue, but I can’t find an existing Bug report.)
I might be wrong here, but thinking about these instances, they seem to occur when (a) the final post in the thread is visible when the thread opens - so either a short thread, or one which has already been mostly read - and (b) post(s) have been made and deleted since the previous visit to the thread (if any).
Navigating away from the thread and then returning fixes the post count error.
The incorrect post count may or may not occur on entering the thread, but appears consistently when the member makes a post. It can sometimes be corrected by scrolling up and down, but sometimes not.