When considering head movement relative to a shot striking the head, You also need to consider another shot also striking at the same general time. Also, remember that Jackie has ahold of JFK's wrist/hand and is pulling Downward and possibly toward herself in order to see what JFK is clutching at in his throat area. Also, for all we know, JFK is coughing or choking at the same time all of this is happening. That shot or shots is Not striking the Rock Of Gibraltar. There is movement/motion involved also.