"Why is having a consistent load factor an enabler for Southwest to fly checked bags for free? Why does it matter that the bags are being moved between the same

"Why is having a consistent load factor an enabler for Southwest to fly checked bags for free? Why does it matter that the bags are being moved between the same types of aircraft?" Having the same airframe means no surprises when loading baggage. This is huge when transferring planes: if both are the same type then every bag will make the transfer, if not, some will and some won't. This enables SWA to encourage checked baggage since they can pretty much guarantee that your bag will make the trip without having to spend additional time at the gate to offload passengers who carry-on all of their bags.