HTTP limit does not reduce sessions

I encountered exactly the same problem, I not sure what triggers the event myself, it does seem to vary according to serveral factors (client browser, traffic level, etc.). I got around it by using CLIENT_ACCEPTED to increment and CLIENT_CLOSED to decrement, this is not 100% reliable but is good enough, I just reset my count every so often (couple of times a month) using the count in the statistics page to keep it accurate.