The feed to the rev counter is a pulse electical signal. On some rev counters there is software to allow you to change from 4 to 6 to 8 cylinders and that can become upset if, for example, the car is stood for a long time and settings memory is lost. But more likely its vibration making the signal even more intermittent and therefor higher reading. In that case its likely the rev reading would jump about a bit.
