The reason the names of the provinces are called old provincial names (kyukokumei) is merely a matter of avoiding confusion between the country as a state (kuni) and the provinces which were within the country; or a reason might be that the prefecture names were used as if they supplanted the provincial names in family register and in mailing address that were used in mail.
国家としての国名との混同を避ける、もしくは戸籍や郵便などの地名の表記において、府県名が旧国名に代わるもののように使用されたために、そう呼ばれているだけである。