Because this surface is made up of many teeth, and the outer enamel layer of the tooth would wear down slower than the inner dentine layer, the teeth themselves, would wear down at an uneven rate.

因为这个表面由许多牙齿组成，而牙齿的外牙釉质层比内牙本质层磨损得慢，牙齿本身会以不均匀的速度磨损。