You're right there is only one Portal server, but there is a second server there. If I recall correctly server1 basically just runs the admin console. It doesn't have Portal running on it.
Quote from some change notes for version 8,
In previous releases, you needed to start WebSphere Application Server server1 before you can start WebSphere Portal. server1 was also needed to access the WebSphere Integrated Solutions Console
http://infolib.lotus.com/resources/portal/8.0.0/doc/en_us/PT800ACD001/overview/change_portal.html