No website, but this thread from two years ago was about the exact same thing, with some pretty good answers.
http://forum.1cpublishing.eu/showthread.php?t=2573
Basically the reason is thought to be the same as here. No reason to have the gunsight 100% in the middle as it performs just as well with a very slight lean to the right. The left side is a little bit less obstructed which favours flying otherwise. The sight itself is only very slightly offset to begin with.