I have seen deer appear to duck a shot from a muzzle loader with a bullet moving at 1800 fps, which is faster than the speed of sound. I personally have never had a deer duck a shot from a bow or a muzzle loader. I always hit right where I am aiming for, or within an inch or two.

I'm not going to say it never happens, but I bet most that say a deer ducked the shot probably miss judged the yardage or the deer was starting to move before they released the arrow. Basically they just missed.