Contact Status

Hope someone can give me an explanation here.
I have my contacts listed by status, online, mobile, offline.
Most of them, when they log out go immediately form online status to
offline. However, i have a couple of contacts that when they go offline,
they still appear online to me, even though i know they are not.
They will usually stay listed that way (online) until i close live messenger
and restart.
Does anybody know what causes that? It's not a big deal, but can be
deceiving.

Thx