Active User Counter keep increasing!

[Coco]

I have a active user counter in my web apps which
previously works but now i realise the counter keep
increasing.
the value of active user is kept in the global.asax which
i do increase during session start and descrease when
session end
i am worried it might be a issues for the server as i use
session to store some data but the counter is definately
wrong becuase i know it's far too high
what could be wrong and how do i resolve that? many thanks!