This is due to a spring and ball on the left side which assures that the sights are always biased to the same position.
Some shooters dislike the look of the angle the rear sight assumes.
To correct this, the ArmaLite National Match rear sight bears a ridge at the front surface opposite the ball and spring. This ridge is used to square the sight with the receiver. The ridge may require fitting. If the rear sight base is too tight to fit into the receiver, or if the base is cocked too far clockwise, the ridge may be stoned or filed down until the sight is squared with the receiver. The raw steel should then be touched up with cold blue and protected with oil or solid film lubricant.