I have wondered about this too. The thing is, head hair is round in cross section and smaller diameter than facial hair. Facial hair is not round. (you can tell by rolling a piece between thumb and finger, or by looking under a microscope). My head hair is 0.002" diameter, my facial hair is 0.005" diameter. A bristle pulled from my cheap bristle brush as not round in cross section and is 0.005" in diameter. Aside form the obvious conclusion that I must be a real pig, it seems that a pig hair (bristle) is a closer approximation of my beard hair than a real hair form my head! Hmmm...