Also, the "vehicle in use" is not necessarily such a clear definition.

For instance, several of my colleagues own historical vehicles from 1960's and 1970's which are "museum registered", which here means that they must be at least 30 years old, can be used only on 30 days each year, and they are not subject to annual "MOT check" (but a different kind of check every 2-4 years).

Are they included in the "vehicle in use" statistic? I don't know. And I don't know what are the specific regulations for similar vehicles in other countries. Because the cars are 40-50 years old, even a not very big number of them would skew the statistics of average age of cars.