This graph shows how accurate law requires speedometers to be- you may be shocked.

The point is that the key measurement you rely on every time you drive is only an estimate, and may not even be a very accurate estimate.

(this reflects UK law; but vehicle designs are don't change much between markets- hence a car can easily be sold in a primary market like Japan and then exported and re-sold as used in a secondary market like the UK or New Zealand where the people drive on the same side of the road as Japan)