This explains it all:
Armalite's Tech Notes, Number 18.
www.armalite.com/library/techNotes/tnote18.htm"1. The rear sight of M16A2 rifles is spring loaded in such a manner that it tends to rotate counterclockwise, as viewed from above. This biasing is caused by a ball and plunger in the left wall of the sight base, which presses against a surface of the receiver and forces the base to rotate. This is a means of taking up accumulated slack in the parts of the sight. The spring loading insures that the sight is always in the same position, even if cocked somewhat to the side. Keeping the sights consistently in the same position aids accuracy."
"2. Depending on the buildup of tolerances, the cocking of the rear sight may be quite noticeable. While this may be somewhat unattractive, it is intentional, and is supportive of good accuracy. All M16A2 pattern sights display this cocking to some degree."
There is more, go to the site and read.
And once again, thank you Mark Westrom for the excellent resource material in your Armalite Library and Tech Notes.