Rpm variates because of the variable load not because of the variable line frequency. It is similar when you drive a car from the city to the hills. You can have your gas pedal at constant but your speed changes. I am not an electrician expert, but I think the line frequency should be constant because it doesn't come from the machine but from the distribution network. Would wait for some other electric expert to tell the truth But the motor should be catching the line frequency not the other way.