Speed tests cheat, they don’t make you download thousands of gigs of data to get a throughput. Which is exactly what you would need to do to accurately test your connection. They send a bit of data, see how fast it goes, do some calculations and assume.

ISP’s can also cheat as they can give ‘burst’ service which temporarily gives you a bump in speed. This can be advertised to you as a perk, but I don’t think many people care, or used to trick speed tests.

It’s like when your boss walks by you and you work a bit harder to appear busy.