Since the main difference between chains 5-11 is the width of the sideplates, it may be that by the time you have .5 wear the thin plates have cut well into the hardened pins to the point that things are going to go south more quickly. While wider chain have more sideplate surface area interacting with the pins, which generally don't need to be as hard, causing the progression of pin wear to go more slowly.

Purely speculation, though.