It is nothing to do really with your broadheads, and more to do with the twist of the string and the helical or offset of your fletching. Almost all factory arrows are fletched with a right helical or a right spin/rotation. The majority of string builder twist their strings to produce a right spin. This way the string and fletching work together.
If you have a right helical and a string that produces a left spin/rotation (or vice versa), then the arrow will start off in a left spin because of the string, and then the fletching will correct the rotation and at some point induce a right spin. This change of rotation will effect the velocity and energy of the arrow.
When you introduce a fixed broadhead to an arrow, then you are actually adding a element of fletching to the front of the arrow which can effect both rotation and planing. That is why bareshaft tuning and ensuring the arrow is coming off the string in a perfectly straight path and in line with the power stroke is so important.
As previously mentioned there are some 2 blade broadheads that are normally used for trad bows that use a slight twist or helical to the blades. But for the vast majority of 2,3, or 4 blade broadhead, the blades are perfectly straight and aligned with the ferrule.
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