Historically, the word "Kokufu" spread earlier than "Kokuga," therefore the word "Kokufu" was mainly used in the eighth century, while "Kokuga" was mainly used after the late Heian period.
歴史的には国府の方が先行し、8世紀にはもっぱら国府という言葉が用いられ、平安時代後期以降に国衙が一般的になった。