APM Inactivity Timeout is not leading to session being closed. It stays open. Any reasons?

. so i have set Inactivity timeout to 300 secs, at same interval i see this timeoutagent.php. But session does not end, ideally it should. what is happening is i see the session expiration increasing by 3 mins every time this HTTP request comes , checking under manage sessions option.But why is that so?