A very poor head would also reach a choke point. Funny thing about MCSA is that it moves around and can even be variable. If flow is not very turbulent, hard surfaces are creating the MCSA. This could be the valve and seat at low lifts, it could be the pushrod pinch at high lift when the valve curtain area exceeds the area of the PRP. Here's the variable part. In an inefficient port, the MCSA is created by 'soft' surfaces. When the airflow separates from the 'hard' surfaces, the separation creates areas of high pressure that shut down the flow area. The separation (or turbulence as it's sometimes called) creates a new MCSA that now controls the flow. Choke would still occur at the same velocity, it's just the choked velocity is occurring through a variable area. This 'soft' choke is what causes the pressures to bounce around on the flow bench.