I believe that the original concept was that the denser the material used in the bridge parts, the less absorption of the string vibration thereby the longer sustain available. That always sounded like a reasonable scientific design. But after 43 years of handling, tuning, testing, repairing, restringing, upgrading, not to mention playing, thousands of many kinds of string instruments, there are a few other factors which have some influence also: The particular piece of body wood (no two are exactly alike), the same goes for the neck, the material the nut is made from, you get the idea. Even the person playing the instrument.