The "new replies" counted when checking on a joined group should be the number of distinct replies that you haven't read. Instead, it appears to be counting the number of distinct users who have replied since you last read. Example: a group gets a new user, and they reply to 6 threads. The "new replies" will be set to 1 instead of 6. Only one of those threads will be labeled as having a new reply, even though you can check the last post time of the other threads to see that they do have new posts.