> So you are really testing the bandwidth of the last mile to your house and the ISP's internal network, which is not representative of your real network bandwidth to the rest of the Internet.
There's no such thing. If you were downloading a file from a friends computer and they have 2mbps upload, your connection speed will be 2mbps. How would your ISP have anything to do with that?
Fast.com just measures your connection to Netflix, which is valuable if that's a service you use a lot.
Yes there is. The Internet is not an abstract cloud with unlimited bandwidth everywhere. Your ISP runs their own network, peered with other bandwidth providers or other ISPs. You may have a 50 Mbps downlink from your ISP, but it doesn't mean the real-world usable bandwidth to all hosts on the Internet will be 50 Mbps, because your packets will be going through these peering links when they leave your ISP's network.
"How would your ISP have anything to do with that?"
Well, simply put some ISPs have terrible peering and/or intentionally throttle. So speedtest.net will show you awesome numbers, but in practice anything you download will be at much lower speeds. This matters, this is precisely why fast.com was built, and this is why the GGP is seeing different bandwidths when testing with speedtest.net vs. fast.com.
Speedtest 12.6 Mbps
Fast.com 13.0 Mbps
Speedtest 40.0 Mbps
Fast.com 73.0 Mbps