This tool exists exactly to highlight cases like yours.
Sounds like your ISP has over-subscribed its connection to whatever IX is serving this to you, and that they're hosting speedtest locally in their network.
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).
Sounds like your ISP has over-subscribed its connection to whatever IX is serving this to you, and that they're hosting speedtest locally in their network.