Atmasphere - I think you are too optimistic on the quality of the US power supply. The US grid is split into 4 quadrants and the supply agreements in place with the power providers have delivery standards etc, but there are NO standards in the agreements on frequency stability. As you will know frequency will vary with load fluctuations on the grid. I have seen figures of 60hz with an accuracy of 5%. This issue is currently being investigated.
Frequency variation does occur but happens over a period of hours rather than seconds, at least, that's what they teach you in school. You may well be right- every time I have checked the speed with a strobe, its spot on, but that probably does not mean a lot as the strobe relies on the line frequency. So all it does say is that the 'table is spot-on relative to the frequency on the AC line. The Speed-Nic thingy says its OK, but IME it does not have a lot of resolution...