PWM + Gargletron + Ring Mod = Octave down???

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Last night I hooked together these three effects that I have built in the last couple of weeks. The chain was like this:


guitar ---> PWM ---> passive ring mod input A
             |--------> Gargletron ---> passive ring mod input B

If that diagram doesn't make sense - here it is in words: the signal was split after the PWM to go into the ring mod input A, as well as the gargletron. Then the gargletron was fed into the other input on the ring mod.

To my surprise, on the two lowest strings of the guitar, it produced an octave down effect, a little noisy and glitchy, but tracking fairly well considering it was totally unintentional. I swept the gargletron filter frequency control, and though it changed the tone a little, it maintained the octave effect throughout nearly the whole sweep, if not all.

Can anyone explain why this is? I actually don't understand how octave down effects work... could it be something to do with the output of the gargletron being phase inverted, then ring modulated (multiplied) by the straight pulse signal from the PWM? That is just a guess, I don't even know if the gargletron output is actually phase inverted.

Also, just an FYI aside, silicon diodes work for the passive ring mod (I have heard that you have to use germanium). The PWM's output was enough to drive a decent enough signal, though it was somewhat below unity on the other end... I have some germanium diodes coming, so I will see what the difference is once those are in.