Granted
Ball out to the skid, in towards the slip. I disagree about airflow. Any airflow that doesn't split the nose is interrrupted by the fuselage and can even cause a slight deflection in the ball, (though not continuous). If this is parsed too much, forgive me. The difficulty is in John's statement that slight perturbations present an upset to the rule, air is fluid, and aerodynamics begs a certain understood starting point, i.e. "balanced", unaccelerated, etc. Crab infers biased flight in a range from 0-90 degrees. It is this bias and necessary drift "correction" that causes the shaded symmetry over the airfoil.
rgds Airfoil