Need something else to worry about? When is the last time you checked the frequency of the power at the outlets? Power companies are responsible for this and usually get it right, but not always. This occurred to me while I was servicing my Generac for the upcoming winter and found it was running about 68 Hz.
I'd suspect your measurement equipment before the power company frequency. Any significant deviation of the frequency would cause the generating station to trip off the line. If the generator outputs are more than about 0.02% from 60hz, the generator risks tripping off the line, being over 10% off frequency would be impossible! All the generators on the grid have to remain lock-step in phase or all **** will break loose! 68hz, that would be impossible within the US power grid, that would force an immediate trip and kick that generator off the grid. Even the tiny fractional variations of frequency are made up over a 24 hour period with TPC (Time Period Correction) of the grid frequency.