I've also run into this bug, and it appears to have just started, so I'm not sure what the error is. Here's perhaps the most extreme example:
It is certainly
possible that 261 users were viewing that thread and 230 guests were, except that I know there weren't because not a particularly popular thread in a not-particularly-popular sub-forum.
Similarly, users are showing up in staff threads- where I know they aren't, since I can test their permissions and they can't get there at all.
Users are shown as viewing threads where they don't belong or don't have permissions, and/or are sticking in threads where they aren't still viewing.