Because we want to, why not? It is my experience that with most bullet sabot combinations there are two loads that will shoot good this is due to the circle motion of the barrel when it whip or if you prefer vibrates. the some time exception is the 44 bullets in a thick sabot in a 50 caliber barrel. I do not know why I just know that it is my experience that that combination don't usually shoot well with heavy loads.
It is also my experience that for long range the heavy loads group better.
It may be like conical the rainbow arc makes it harder to place the bullet accurately with out a great deal of practice.