The error is most likely caused by some noise on the power lines that the EMS is somehow interpreting as the incorrect frequency. The power supply from the electric grid is quite well regulated in terms of frequency, within 1 or 2 Hz at the very worst under normal conditions (instantaneously) and within a tiny fraction of a percent when averaged long-term. The long-term accuracy is due to the need to keep clocks that reference the power line from gaining or losing time on average: a 1% error there would mean their gaining or losing about fifteen minutes over the course of a day, which is clearly not acceptable.
The main problem you might see if the frequency were actually well out of specification are some clocks and timers running fast (or slow)--these days, that's mostly mechanical appliance timers, as digital electronics generally use a crystal oscillator or similar timebase independent of the line frequency. Synchronous AC motors will try to run at a different speed, but for most household devices that's not a concern as their actual speed is usually a result of mechanical loading rather than synchronous line frequency. It would take a quite large deviation to cause "real" problems that might damage things.
