The hour meter counter part of the tacho is calibrated to clock over 1 'hour' in a real hour at a certain RPM - typically cruise RPM.
If the revs are less than calibrated then the counter ticks over proportionally slower. If revs are greater then it ticks over faster. Similar to an electricity meter.
In most flights you will average a lower RPM (for the whole of the flight - start up to shut down) than the calibrated RPM. End result is that the counter doesn't tick over an hour in a real hour but typically .8 to .9. Shorter flights at low power settings gain more than longer flights in the cruise or at high power settings.