Vanes, fins... whatever you call them, it's all mold flash. This is caused by the mold faces not closing tight together. This is usually caused by a piece of lead spatter, or a dinged place on the face area of the mold. It could also be caused by some piece of debris in the locator/alignment pins.
Whatever the reason, it's not good. When your mold is flashing, you are molding out-of-round bullets. Granted, it might not be MUCH, but it is out-of-round.
Get some opti-visors, or a good magnifying glass... or, if you have access to it, a microscope, and inspect the faces of your mold. Carefully clean any debris or spatter from the mold, then try opening and closing it again. Keep after it until you are certain there is nothing hindering the mold from closing completely. Then mold some with it... the flash should be gone.