Almost all of the other replies are excellent. In light of your question I think you'll find interesting this realtime map that shows how far out of whack the regional grids are from 60Hz: http://fnetpublic.utk.edu/frequencymap.html
Seems like there's plenty of deviation in North America, more even than the European grid is complaining about. Quebec and the Maritimes seem to oscillate between low and normal, and the west seems to always be a bit too fast.
Yeah you're seeing the short term variation at that site, and there's plenty of it. But over a 24 hour period
- for now anyway - NERC standards require time error correction, http://www.nerc.com/files/BAL-004-0.pdf
Not as cool a visualization, but https://www.swissgrid.ch/swissgrid/de/home/reliability/wam.h... shows the frequencies and phase differences (in degrees) between a few measuring stations around Europe. The phase difference between Switzerland and Spain is now some 50°, which is actually much larger than I would imagine these differences becoming.
Aren't all the grids in US/Canada also connected? My understanding was that they are all connected and that the grandparent visualization was showing the frequency differences across major portions of the US/Canada grid. Or am I getting this wrong?
No, they are not. There are four major grids: West, East, Quebec and Texas. Each one is referred to as an "interconnection". Synchronism is maintained within each interconnection, and they may have non-synchronous (i.e. DC) links between each other. Here is a map:
