Actually it would appear the session isn't deleted upon sign out. Or the who's online list is cached and not updated on sign out. Cookies are cleared or you wouldn't be able to sign out.
This is actually important to some of the members on our site, it would be really great if people no longer were displayed online after they have logged out.
It will probably be changed, yes. It's quite important that the data lives on after you sign out or at least the reason it's kept around needs to be handled before removing it, so thus it's not trivial.