A speedometer is on the technical side a rev meter. As you can change rim and tyre size (e.g. summer and winter tyres), the whole circumference changes, leading to different speeds that the car can't measure. So with summer and winter it shows the same speed but in reality, you're going different speeds.

As typically by law, the speedometer is not allowed to show too slow, it's showing by design too fast. So that even after changing tyre size, it will still not accidentally show too little speed. It's necessary due to the measuring method with unknown variables.

Lorries typically have a trip recorder for speed and rest periods. Their speedometer is usually calibrated, which is also why people often think they are speeding when comparing to their own car's speedometer. But it's the car showing the wrong number.