Because the blades on a fixed blade head are trying to "steer" the arrow. The vanes or feathers are also trying to steer the arrow. If they are working together instead of opposite one another, accuracy and flight efficiency are enhanced. It's just physics.

Check it out at:
web page

Take an arrow and spin it in a wind tunnel and watch the airflow. If you think the flow off the spinning blades is coming straight back across the vanes you must have had too much ridge ripple.
If your bow is tuned properly, the blades will NOT steer the arrow.
Maybe if you extend the blades all the way back to the fletch they will work together. Real forgiving to rests that way too.