The highest mountain ranges are formed by geological forces that raise the earth' s crust: two continent-bearing tectonic plates of comparable density collide and crumple upward, causing a thickening of the crust. The erosive forces of wind and precipitation inexorably wear these mountains down. Yet the highest mountain ranges tend to be found in places where these erosive forces are most prevalent.
The statement 'The amount of precipitation that a given region of the earth receives may vary considerably over the lifetime of an average mountain range' most helps to reconcile the apparent conflict described above.