The ISPs and Carriers have been setting up Speedtests on their private networks which gives inaccurate internet results. Select an outside speedtest location and the speeds generally always drop. IMO their interconnects are the bottlenecks. A few tests from other ISPs have found faster speeds to the same speedtest sites. Traceroute has shown fairly direct routes in some cases, convoluted routes in other cases.
When it comes to multithreaded routers with PCs operating faster than 200Mbps, Speedtest dramatically under reports. DSLreports.com/speedtest is much more accurate in that enviroment. In the past T-Mobile was caught by the FCC giving higher priority to speed tests iirc. Journalists have taken to random movie download times to test 5g.
Send me a diagnostic report next time you run into that.. but based on what is on the screen, that looks like a legitimate T-Mobile connection. My Pixel 4 has a hard time letting go of T-Mobile LTE in some strong Sprint areas as well.
As an update to the 5G discussion -- I have received several diagnostic reports from users who noted they were connected to 5G at the time (thanks!), however I have not seen an example where the API is reporting any 5G data. Everything has appeared as regular LTE for now.
Please keep sending me reports, I am sure eventually we will have a device that reports it properly! At a minimum it will need to be on Android 10, because the 5G reporting routines did not exist in earlier versions.
No, its because Speedtest has considerably more servers to ping (meaning less hops and closer in proximity) from than any of those other servers you mentioned. Typically, the fastest speed would be considered the most accurate.