the clock has actually run slightly fast, gaining about a minute a month relative to atomic-clock references.
Actually it is a chrystal that regulates the pulses from which the time is "calculated", the cheaper the chrystal is, the more change you have that the time will show a little difference to the "official time messurements"

If you want a very accurate time you need to have chrystals as organisations like NASA are using and highly reliable electronic components which will make the mobo’s a lot more expensive. So manufactures are using “the middle road”. Accuracy/price.