So the other day, my internet was very slow, I did a speed test on speedtest.net, and it reported about 25Mpbs, which is exactly what my internet is supposed to be.
But it didn't seem right, because every site was painfully slow. So I went to speedof.me and did a speed test, and I was getting like 1.5Mbps, which was definitely more accurate.
The other day I overheard a Comcast commercial where they mentioned that they're number 1 on speedtest.net. So, I came to the very obvious conclusion that Comcast deliberately makes sure to un-throttle your internet when doing a speedtest on speedtest.net (and who knows where else).