Not completely true. AI have understood so far that density of 0.30 equals 1 stop.changein density of 0.3 equals 1 stop. Furthermore this is true only for a H&D curve that approximates a straight line with a slope of 0.45. Obviously such H&D curves are not straight lines over their entire length. The H&D curve is a semilogarithmic plot for which the log_{10}2 = 0.3.

So what you espouse is a glittering generallity which is only partially true. As sugggested above read up on the H&D (characteristic) curve and things will be clearer.