APM session inactivity timeoute VS. TCP idle timout
So APM works on logged in sessions, and users but from that the application might open many tcp connections to service that session.
So really its contextualisation of what APM is doing (Access) and ltm is doing (connections).
So your timer question, many tcp connections may have been opened and closed but you have never logged out of APM therefore your session is still open.
Hope that helps, i've had to fault find APM one or two times and its always diffcult trying to get it all to line up.
But that's more the application and how it works rather than the f5 APM module.