You need to go back and read your contract. They likely promise you nothing. It likely say you can get a maximum of 20m which you do you can actually get 22m. Unless you pay for what is called committed rate everything is goes back to best effort. Some do not even promise a minimum rate.
This is just like the scam a lot of the cable companies have with their "turbo" option that allows you to download faster for small files but then slows down. This is really used to cause the test site to report higher speed since they test only for a short time