I'm gonna say that much of the inconsistency can be fixed while selecting the ribs at the store. 3 slab packages with the mystery slab in the middle are an enemy of consistency. Also look for ribs with small streaks of fat that travel through the meat. Big sections of red meat with little to no fat running through them is a sign that you might want to avoid that slab.
I have found the weight of a slab to be unreliable when trying to get them the same size. I look more at the physical size and shape, as well as the thickness. Sometimes two slabs will weigh about the same amount, but if you inspect closer you may find that one has larger bones and is cut differently.
Hope some of this ends up being helpful.