We need to hear from Shaw on reasons for differences in flight--mechanical vs. fixed broadhead vs. field point
it all has to do with increased surface area and wind planing.
a field point has less surface area than an expandable and an expandable has less surface area than a fixed blade. you can get away with a little more error in tuning with an expandible but with a fixed head, if your arrow is coming out of your bow crooked or unlevel then the blades of the broadhead will steer the arrow in that same crooked line. conversely if your arrow comes out of your bow slightly crooked when shooting a fieldpoint your vanes will quickly stabilize the arrow from the rear and send it on a straight course because the field point has very little surface area and cant steer the front of the arrow. think of it like, how could you steer an airplane with wings, how would it steer with very small wings, and how could you steer it with no wings.
edit, I know im not shaw but this isn't rocket science