Why, because Tom House says so.
Just kidding. Just thought I'd jerk someone's chain. :mrgreen:
Actually, I know the answer for possibly the most common scenario. First, many pitchers have been coached to start on the throwing arm side of the rubber to create angle. Second, many pitchers stride to the throwing arm side to various degrees. In cases where either or both of these conditions exist, the pitchers' drag lines will often extend to the throwing arm side of the centerline of the rubber. Also in these cases the pitcher will do something to get themselve back inline with the target. Often times that something is a late posture change whereby the pitcher bends at the waist toward the glove side. If the pitcher moves so that his drag line ends on the centerline of the rubber, he can reduce or eliminate the posture change.
For cases the above conditions exist but the pitchers drag line actually extends to the glove side, I'm not sure exactly what the NPA's recommendation would be. I had such a pitcher and I positioned him to eliminate any posture change without regards to the drag line.