Mostly it's desireable to try to balance the tail area around the line of flight. A high mounted rudder creates a slight adverse rolling action. A low mounted rudder a proverse rolling action. The CAP rudder with it's strongly wider chord below the centerline and narrowing as it rises tends to balance the area so the rudder has a more or less neutral rolling side effect. Pattern fliers play with this aspect a lot in their designs to try to achieve pure control effects with as few side effects as possible. The rest of us don't really worry about it much.