A partial reason for this is that the speedometers have to be optimistic by law (depending on jurisdiction, but I think this is the case most everywhere. When

A partial reason for this is that the speedometers have to be optimistic by law (depending on jurisdiction, but I think this is the case most everywhere. When your speedometer shows 100 km/h, it is permitted that true speed is 90 km/h, but it is not permitted that the true speed is 101 km/h. And the same variance applies to odometer, of course. Things like tyre wear and pressure have a noticeable impact on what both meters show, and manufactures have to stay on the safe side, by law.