I don't use BFD, but with my Roland module I have sometimes seen this: I assigned a snare drum to one trigger, then assigned the same snare drum to a different trigger, using a different note number. So even though I am using the same snare drum sample, I am actually using two different snares, because the same sample is assigned to different note numbers, and therefore settings for them can differ.
What I am trying to say (and doing a poor job of it I think!), is that the Zendrum triggers may have nothing to do with the volume difference at all. For me, every time I thought it was the ZD triggers, it was actually the module. Not something wrong, just 'operator dumb-dumb'. After much exploration and head scratching I finally figured it out. Course I keep making the same mistake over again, lol !