No ISP on the planet can guarantee speeds across the internet. They can only guarantee speeds on their own network. I suspect most people that use speedtest.net understand this as it quite literally shows that your ISP is hosting the test (assuming your ISP has one).
Really? I've never seen my ISP listed on that site or seen in any obvious way that my ISP is hosting the test. Maybe it's there but if it is, it's either hidden, or done in a way such that it's non obvious your getting "fake" speeds compared to the rest of the internet. There's no disclaimer that because it runs on their servers you'll get higher than normal speeds in the test. And in my experience where I live the average consumer has no idea those tests are bullshit.
If it's actually a test of internet speed, why are they choosing to run it in a way that does not reflect the true speed you will see when using the internet. I remember reading a reddit post where a person said he noticed that the speed tests were ridiculously faster and thus decided to proxy all his internet through that address. Anecdata but it was stated it _vastly_ increased his internet speeds until the ISP caught on and stopped it somehow. The question was how they might continue to get around it.
It's obvious the ISP are not providing even a fraction of the advertised speeds to an advanced user but average people tend to really believe they will see those speedtest speeds throughout the rest of the internet.
"the true speed" isn't a thing for internet. All speedtest services just measure speed for specific server.
fast.com is notable speedtest service because it measures between Netflix' server, I recommend it. It reflects real usage. If your use case is torrenting, just download popular Linux ISO.
