there should be a continuous gap for session to expire.
suppose 10min is the expire time.
suppose i open a page. i refresh the page after 6 minutes then after 5 minutes. its been 11 minutes since start but only 5 min since last visit, so session will expire only when its been 10 min since last refresh.
if i fetch the page again after 11 minutes instead of 6 min, the session will expire.