Yes addicted I have noticed that to. I think when you sign off, the site still shows you as on line fore a little while. There is probably a time period that the program has set to reassess it's data. So if you sign off a second before that timed update then your name disappears immediately. If not you have to wait for the next processing period. It would be interesting to know how long that period is.
Similiar to when you are on email full time, your machine checks your IP on a regular basis. I know mine is set to 5 minutes.