so after a couple of minutes of googling, it seems like my initial thoughts were right. tremolo effects are based on using an lfo to modulate volume. for example (also thinking about the zvex tremolo-probe):
http://www.visionwebhosting.net/ireland ... ng0151.htm
or
http://www.hammersound.net/hs_sd_vintage_synthesis.html
this makes sense to me with guitar because a lot of tremolos have a "depth" parameter that controls how much the lfo modulates the volume and then different lfo shapes create different tremolo/volume moduation effects.
the ring mod at some points uses an audible carrier (which you can plug into in amp and use as a sound source), but also goes below that (the lfo and the carrier frequency overlap some) but that doesn't really matter for the tremolo-like effect. the 102 manual says that the tremolo effect still has two sets of pitches, they're just close together and you don't hear them as different pitches but they cancel. mixing in the dry signal would add a third pitch and so the cancelation is different (more complex?).
so even though tremolo/ring mod sound similar, they don't functon the same way. does this help?