1 pointTestmy.net has an extended auto bandwidth test with frequency as well as duration variables. I would imagine the throughput spread your experiencing is due to the ISP testing server is likely located on their system, close to you, before any external network or shaping / filtering they may have in place. Where testmy.net is testing the route between your ISP and the testing server.
1 pointGreat article on Motherboard. .. https://motherboard.vice.com/en_us/article/d3ewvq/study-shows-how-badly-most-of-the-country-is-getting-ripped-off-by-their-isps How much do you pay and what are your speeds?