Aiming center is moved on the right side, this offset changes if you change shooting distance. It is not like tanks where you have Optics not centered with cannon.
Explanation video: https://www.youtube.com/watch?v=6jCDyitzOSI&feature=youtu.be
It is similar to Arco Optics aiming problem reported here: http://feedback.arma3.com/view.php?id=19257