I am glad I got called on that because my wording could have led someone to believe I was referring to trajectory when I made reference to the " arch" of the arrow.

Arthur,

Good question and I am glad you brought it up. Your logic is sound and I would assume much the same without actually seeing it. However, the width of the curve of the arrow in relation to the width of the fletching allows the fletching to be " offset" to the true center of the arrow. In other words one could consider it parallel to a straight fletch. Since the arrow surface is relatively wider than the fletching width you can still place it parallel to a straight fletch on that portion of the surface of the arrow while still not turning it into a straight fletch but from a different angle.

The picture above is somewhat helpful but I am going to see if I can put one together myself.