Need help analysing experiment results

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 red real response vs the blue 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 shifting up and down in a sinusoidal path, does anyone know what this means?

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