@Junos - One problem: For the lowest mouse setting, if I move the mouse very fast and in large swipes, I have almost no movement on screen, as opposed to moving the mouse slowly, on a higher sensitivity setting - LO and behold, I am turning faster.
So no, it has nothing to do with realism, as it gives a clear turning advantage for high sensitivity low mouse movement users, or it is as close to 1:1 movement the less you move the mouse.
[EDIT] Just checked. As of the latest build (0.35.115188), you have 1:1 mouse movement while aiming down sights AND on vertical movement. Realism dictates that I have SLOWER movement while aiming with a rifle than a free head bob, so it IS a bug and not a feature right now. Might as well give 1:1 until it BECOMES a feature, rather than an inconsistent mouse movement bug. Also, there is nothing real in not having 1:1 in a forward 90 degree arc anyway, at ANY speed.