During peak demand the main frequency may be 59.687 Hz for example, for say a couple of hours when all generators in the system are heavily loaded and it is not warranted to start up extra generating capacity.
I don't think that they normally let the grid frequency drift that much. Under normal conditions the grid operates at plus or minus 0.01 hertz. When time correction is needed for cumulative error correction the system is run at 60.02 or 59.98 hertz.
Don
