Mine had a false address posted... hmmm... now did I spoof it or was it just needing a refresh???
It would show the address that the web request came in on. If you're behind a home router/modem or hardware firewall, the site would see the IP address of the router/firewall instead of your actual PC.
If you don't have either (or see a different IP than your router has), then either you have a proxy server specified in your webbrowser, are using an 'internet accelerator' product from your ISP, or they have you behind a transparent proxy to save a little on their bandwidth cost.
You may be able to find out more by doing a reverse lookup on the IP address that gets shown.
e.g: nslookup <ipaddress> to see the name of the device associated with the IP in question.