I'm a Comcast customer. When I run speedtest & tcpdump the traffic, i'm nearly always talking to something .comcast.net, and it is just a few hops away via traceroute.

When I use fast.com, I'm talking to various ix.nflxvideo.net hosts, all at least 4 to 6 hops farther away.

The amusing thing is that I get better speed with the fast,com (140Mb/s) than with speedtest (133Mb/s).