1. It is normal. Some frequency/modulation responses from the tube are different than others.
2. As long as the communications is happy between the generator and the Central unit, and the bulb is flashing on cue, I say it is normal.
3. In this example, a larger frequency is modulated by a lower frequency. Note how the larger frequency (depicted as the carrier signal) is encapsulated by the lower frequency, and this changes the amplitude of the larger frequency at various points. In your case, the SC is using square waves, so the resulting waveform has a different shape, but the principle is the same.
For more information please check this link: