For what it's worth, a popular speed test site could gain kickbacks from ISPs who want their customers to see "correct" number.
However, the deal is mostly not about speed test sites but about ISP behavior. ISP may maintain a registry of known speed test sites and shape traffic so that you get great results on those.
Registries aside, your ISP may also be detecting and throttling BT and/or VPN traffic, shaping sustained traffic, etc., meaning you will tend to get good speed test scores but get slow speeds if you attempt to use your connection in a real-life scenario.
And back to speed tests, they do tend to prefer servers located very favourably for ISP but not for you, routing to server on same ISP's network in the same city by default (this may be overridable by a tech-savvy user). I would assume they have agreements with ISPs on hosting and maintaining those servers, making any sort of manipulation a no-brainer for the ISP.
All in all, I routinely get cases where a speed test reports great results but due to whatever reason I get really slow effective speeds.