Balance issues?

 After playing around with a little aircraft creation for quite a while, trying to balance it perfectly, I found that even though the symmetry is perfect at the sides, the aircraft leans to the left. All rockets have the same power and the "weight" should be the same at both left and right. It almost seems as though the robot or the chair is imbalanced.  I uploaded a little video to further clarify my point. Any info on this, and is this a known thing at all?  

Video: http://youtu.be/ws0fWmqiChM

PS. I am aware that I can just control the rockets on both sides with hotkeys to compensate for this, but I was hoping to build a fully automatic plane for this level!