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I can only speak for the UK but I'm sure it must apply in other countries: The 50Hz is the average over a long period (maybe 24 hours) so that line powered timing devices, for example clocks will keep accurate time. When the demand on the power stations is high, the generators are under heavy load and they slow slightly, to compensate, they are speeded up at times of low demand so that the number of cycles is the same each day.