20mph might be a moving average speed; my cyclocomputer shuts itself off after a few seconds of inactivity, so time that I am stopped at a light is not counted in the time that goes into the distance/time=speed that is reported as my average.

Also, they might be reporting something more like 'mode' than mean (aka average); meaning, when they're not impeded by intersections, their cruising speed is 20+mph. So most of the time, the first digit on their speedo would be a 2. But if you factor in stops, and slower speeds for braking to a stop and accelerating after a stop, the overall average speed would be less.

FWIW my bike computer very consistently shows a 10-11mph (moving) average. I can get over 30 on downhills, and crawl up anywhere from 3-8mph depending on the hill. Typical cruising speed on the flats with neutral wind is probably like 13. But I am definitely NOT a fast rider. I get passed all the time by people who care more about riding fast. I'm sure I get passed by at least 10x as many cyclists as I pass.