Yea I noticed that years ago but never payed any mind to it.. I guess the amp dont pick it up cause its too weak to pick up as it wouldnt travel along the string like tapping a note.. as we know when u hit a note the string vibrates along its self, but this seems more like the string just hitting the fret very slightly not enough to register as sound for ur amp to pick up... its much more pronounced on an acoustic guitar.. but havent a clue what its called ..