The real-time activity monitor is the full page that automatically updates to show the latest posts. The pop-up box, on the other hand, is what notifies you of new replies to tracked threads. Assuming you mean the latter, it displays only when a new reply has just been posted, or occasionally if you have just returned to an active state within the forums after having been present when the reply was made. It's not influenced by 'receiving a hit.'
It seems like the box pops up whenever the site in question gets a hit, not necessarily when somebody replies. Is that the way it was designed?