Uncirculated mint sets are assembled using coins which have been poured (yes, poured--in bulk) into containers (bags, etc.) for use by those who package the set. Lots of jostling and coin-to-coin contact. When one considers the journey freshly minted coins take after they are struck, it's no wonder they may have hits, dings, and scrapes before they get very far. Coins used in making the uncirculated sets are not lovingly carried one-by-one from the striking chamber to the set packaging room. Over the years I've had set coins appearing anywhere between weak MS60 to strong MS67--with the vast majority falling around MS64 or 64+.