Hmm I wounder what causes this.I just looked at the diagram and the air flows through the evap over the heater core and in the vents as long as the doors are open.The only thing I can think would be the freon is turning to a liquid half way through the evap there for cooling half the air. And there must be a wall in there the separates the air flow.So the passanger side air dosent mix with the driver side vents.