I've noticed the same thing going back into the Statehood Quarters and pondered on the same oddity. Researching the matter, the U.S. Mint contracts out to two private companies for the nickel through dollar planchets. Olin Brass of East Alton, IL or PMX Industries in Cedar Rapids, IA. So this would suggest a slight variance between suppliers. I could not find any information as to which mint orders from which supplier, or if a combination of the two. Either way, we may never know.