UA actually deliberately exploits this "feature" by frequently opening only a single seat in the lowest available bucket. It can be extremely frustrating, as the fare jump can be substantial, particularly when going from say Z to D.

While it's true that having a single PNR with different fare rules could in theory add complexity, in many instances the only difference between the two adjacent buckets is the price. By definition, you're already meeting substantive purchase conditions (such as AP).