As far as I can tell there's a bug in the software so it doesn't actively check if you're still in chat when the total number is down to one. If you log out of chat like you're supposed to, it seems to clear alright, but who bothers? Most people just close the window or log out of ukCaving. Then the chat software doesn't get notified and the bug strikes, leaving the last person to use chat apparently still there, like a ghost...
What Graham is seeing is the probably the same thing. If Graham is the "ghost" and I log in, the software suddenly sees that there are two users in chat, springs into action and does its normal checks. Lo-and-behold, Graham is no longer there. So it tells me he's just left.
I imagine it is a bit of scrambled thinking on the part of the programmer. There will be special considerations when there is only one user left. For example, it may be that the software stops checking whether Graham is still there once he's the only person left, because there'll be nobody left to tell when he finally leaves. It would be quite easy to overlook the need to check anyway for the sake of the "current users in chat" and "Elvis has left the building" features.