That assumption is right. But until now it was always compensated, so that in the mean it would be ~50hz. This correction mechanism apparently isn't enough anymore for the current fluctuations in europe.
Not mean, average. That's a very important distinction because if it was mean, then it might never correct, but it does.
It's supposed to average 50Hz over time, so using it for clocks make perfect sense.
It can be allowed to fluctuate, but still regulated to keep pretty much exactly 50Hz mean value over time.