It's stated in his character sheet that his reactions are in no way a normal human. You also assume that he'd be taken completely off guard that his opponent would think of dodging. I wouldn't think I'd have to edit into my post him planning for every eventuality that his opponent might do or else say he would be caught off guard by it. Even I, without any military experience, knew that he'd probably try to dodge it.

If the mech is still swinging the weapon around wildly until Saanga hits the ground that it would make sense that he couldn't hit it, but it seems like he stopped to aim while Saanga was still in the air a bit. If it wasn't for but a fraction of a second that he started to aim then this makes more sense, I thought the mech did his running, turning, and aiming a lot quicker than just being able to run and turn while Saanga was in the air. Hitting a target while you're moving is easier than hitting a moving target. I assumed the gun was bigger than it actually was.