Can't see how they find this over the web.  I'm on Windows, its in my network config.  It changes when I use a VPN provider, but not when I use a local socks proxy connected to a remote server over SSH (ProxyCap).  None of the publicly available sites are finding my correct IP, but leaktest finds my ISP DNS.

I imagine the protection is in using a VPN that is creating a virtual network adapter and therefore setting DNS providers there, but how is the website detecting my provider?