Burris is absolutely dead on, always check the durometer of the tires at a certain air pressure and temperature, otherwise you'll get a different reading every time. I check all of mine at race pressures and at room temp. You might notice that the durometer reading gets higher as the tires get thinner also, its because the durometer is actually hitting the chords in the tire rather than the rubber itself, which will cause some high readings in certains spots of the tire when its worn. When tires get that worn out, i give them away or throw them away and get new ones. Softer duro doesnt always mean more grip or bite either, thats another good point.
002, i believe its the materials the tires are made with that gives them more bite or grip, such as more natural or synthetic rubber. Im sure Burris Racing can verify that and give you better info though, im interested in that answer myself.