The caliper is mounted on a Swing bracket. At one point the caliper is actually perpendicular to the rotor, and pushing straight on. Further in or out the caliper is at varying angles to the rotor. This spherical washer, takes up the difference. The back of the stationary pad should have similar bump on the back. .
This is how it theoretically works, but dont be too surptised if pad wears at a slight angle.