The secret here is that this is measuring speed to Netflix, which on many ISPs is throttled well beyond what other speed tests will report. I'm in the Boston area and getting 100Mbps on fast.com, but getting 250Mbps on speedtest.net. Fast.com is even encouraging you to compare their result with speedtest.net.

It's a pretty smart tactic. If you're paying for 150Mbps, but being throttled to 100Mbps on Netflix and your ISP doesn't tell you, you're not really getting what you pay for.