It's magic! ;-) All kidding aside, that's not really how our proxies work. You can find the technical explanation in this image: http://i.imgur.com/fkWiQyv.jpg.
So basically you connect to our access server which forwards your request to your unique final IPs. The IP you see if the IP of the access server.
Wether you use a unique IP and static port (say, 80 or 8080 which are common), or a static IP and dynamic ports (like with us), it's the outgoing address that counts.