Counting Number Of Users Online

I have a GLOBAL.ASA file in my Website directory :

Sub Application_OnStart
Application("Online_Users") = 0
End Sub

Sub Session_OnStart
Application.Lock
Application("Online_Users") = Application("Online_Users") + 1
Application.UnLock
End Sub

Sub Session_OnEnd
Application.Lock
Application("Online_Users") = Application("Online_Users") - 1
Application.UnLock
End Sub

When I open a new Session (by opening an IE window) : Online_Users has increased.
But when I close this window, Online_Users still remain, not decrease.