I certainly see the behaviour you describe where the XP notification is shown for any number of pages opened within a small (few tens of seconds maybe) period of time, including refreshing a page within a small period of time. When it happens it seems to affect all pages opened within the period. As you suggest the effect does not happen consistently and I can't put any sort of reliable time period on the "window of opportunity" except that it seems to be more than a few seconds and less than a few tens of seconds.