At Home: I have a LAN and when I start the Windows, I have always connected, so the avast! will update as soon as possible after start...
At University: After I start the Windows, I don't have any connection, so I have to login to the wireless. Between the Windows start and the login, the avast! will try to update, but as I don't have any connection, the avast! only try to update again 2h later...