If you're using never expiring autologin and "session gc" cron task doesn't run for some reason while you're offline and your session is expired (say, run very very rarely, or doesn't run at all - switched off, or on boards having too few users, or so), user_lastvisit field won't be updated because your session will be constantly updated and valid.
In this case, your lastvisit time can be updated only if session gc cron task was running while you haven't been online and your session was expired.