Need help analysing data

So I’ve done an experiment using solenoid valves controlled by an Arduino to move a hydraulic actuator. The experiment was for the movement of the actuator to match a sinusoidal input produced from a signal generator.

OnOff sinu

Looking at the picture below you can see the response vs the input at three different frequencies. Can anyone tell me why the response maintains a constant amplitude at each frequency but its mean value keeps on moving up and down?

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