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

Going forward it may not be a good idea to rely on the grid's timebase as there's talk as well as an NPRM out there to do away with the standard. https://www.federalregister.gov/documents/2010/03/29/2010-64...