The curvature of a progressive addition lens surface gradually increases toward the bottom of the lens, becoming increasingly steeper. This increase in curvature (and surface power) is what produces the add power of the progressive lens. Unfortunately, because the bottom of the lens is steeper than the top of the lens, the upper edge of a progressive lens blank is thicker than the lower edge.
Understanding Prism Thinning (pdf)
by Darryl Meister