I 'think' the small amount of distortion created by tightening the hardened bolts,with their specifically sized and machined area under the head, may help to create a locating effect that is normally accomplished with a dowel or tapered cone. Both of the last 2 would add additional machining steps and cost.I think a hard steel washer would elimate that 'freebie' effect mentioned in the beginning,as it 'may' act as a bushing between the 2 mating surfaces. Not sure if a dowel is included in the equation. That spinning flywheel creates some serious energy there,especially when try to move a 4000lb vehicle with sticky tires. Conical lug nuts on a street car comes to mind.Nothing textbook here, purely speculation and observation. Just my thoughts. Bill