Well, I think the phenomena that exists in the fact that, the AIM server sends the incoming messages to the windows box possible because it was the first box to 'check-in' with the server. Also, the AIM server is designed to log the user if he/she logs-on from a remote location. It's naturally due to the fact that GAIM and AIM don't work the same or use the same packet structure, or something of that nature.

scat