I don't know 100% how it works but some of the waveforms you can choose are mono and others are stereo (on the X knob if I remember correctly). I guess if A and B modulated exactly the same way the effect wouldn't be as interesting, especially in stereo.
I know the X knob function and the stereo waveform settings but as already mentioned delay A has always a deeper modulation than delay B and delay B doesn't modulate at all with a 0ms setting no matter which waveform I choose while delay A always does…
I think user documentation about how algorithms actually work in lower end effect units has always been very poor to be fair.
I hope eventide will improve that in the near future!