There are different things that skew the clock. Some PCs have clocks that run too fast or too slow. Sometimes the environment or source of power itself will influence the speed of the PC's clock. Changes in room or machine temperature will do this too. Ie. if your room swings from 70 degrees in the morning to 80 degrees at high noon, this will influence the clock's speed, likewise, a hot graphics card or cpu from heavy processing can do it too.
Even with NTP and a reliable external temperature controlled time source or connection to an atomic clock, if you correct the clock on your PC every second, or every 10th of a second, the time that your PC produces can look more like a jagged saw tooth pattern than a straight line.
The best solution I've seen so far for PCs was a third party application called "PresenTense Time Client" by Bytefusion that adjusted the algorithms for slowing and speeding the clock so that it produced a more sane representation of the current time.
