The other factor in this subject is gun tube life. All gun tubes had a certain service life expectancy. One of the reasons Gribeauval designed his gun tubes the way he did is that he didn't like the relatively short service life of either the current Austrian or Prussian gun tubes, both of which he had tested.
Your point is well-taken. The gun tube life was calculated by how many rounds could be fired before the gun tube gave out. Both canister and round shot gave wear and tear on the gun tube.