Every once and a while I poise myself to take an unsuspecting life from afar, placing the center of the crosshair dead center of the poor soul's chest, only to have the arrow completely avoid his or her silhouette. I know the arrow jumps up a little if you're too close, but I don't see any reason why it would swerve left or right when I'm standing still. Is there a hidden mechanic here, or am I just strafing unknowingly?

Did you account for the wind that doesn't exist?

Did you account for the Coriolis effect?

I did not, but skyrim being the northern most province, coriolis effect should be minimal =P