It you take the trouble to look inside the small drop down flap to the right of the driver you will see a small interface socket that gives access to the vehicle's ECU. From here it is possible to plug in any laptop equipped with the right diagnostic software to alter many engine parameters and the odometer read out.
I gave no indication that this was wide spread, only a number of possible occasions which may give rise to a higher than expected reading.
Regarding the legality aspect - catch me if you can. Changes, if any are generally quite small and in no way attempt to alter the sale value of the vehicle, as it has already be sold. How ever should this happen after the initial sale, and can be proved then thats a very different matter.