There is a difference because these two measure different things.<br />Speedtest by Ookla is measuring the bandwidth available from your site to a single Speedtest server chosen to be as network-close to your site as possible, across a single connection. It uses fairly small packets and measures the latency of the connection once, at the beginning.<br />Fast.com is measuring the bandwidth available from your site to one or more simultaneously-connected Netflix servers that are of varying network-closeness to your site, and also use multiple simultaneous connections to each server. It continuously measures the latency both unloaded as well as under bandwidth load.