ParilIt's very browser and user dependent.
The chat relies on "unload" events to tell the server to log you out of chat, which is problematic for many reasons:

1) unload events are skipped if they take too long, or sometimes not called at all, in order to keep browsers moving smoothly
2) chat is in an iframe, which further makes 1) worse
3) by closing tabs & browsers you are forcefully cutting off the connection where it was left; you should always browse to a different page instead, as switching pages lets the old page destruct properly.

Soon, chat won't be in an iframe, which might fix the issue. The check was implemented by Cyprezz to help burden the issue of people having multiple chats open, which doubles their bandwidth to the site.

-P