The exerpt from the FSM in my previous post says that "engine mounts differ with regard to shore hardness", so you're probably exactly right. Shore hardness is a way of quantifying the hardness of the material.
http://en.wikipedia.org/wiki/Shore_durometer
I'm curious just how quantitatively different these parts are, but I suspect that the shore of the rubber in these things is not something the competing manufacturers advertise.