You're forgetting one very important thing my friend. It is true that the link fastened to the servo arm moves less per degree of servo movement the further the arm gets from the 90 degree point (in both directions) BUT the item that the rod is connected to at the other end works in exactly the opposite way and so the rotation output more or less equates to the rotation input.
True only if the distance between the spline and the ball link is the same between the other ends ball link and its pivot point. Otherwise you still introduce expotential movement.