For the Torah of Moshe Rabbeinu appoints bnei Adam as Kohanim Gedolim, bnei Adam with frailties, but the dvar HaShevu'ah [Tehillim 110:4], which came later than the Torah of Moshe Rabbeinu, appoints HaBen [Ben HaElohim Rebbe, Melech HaMoshiach] who came to shleimut  completeness  l'Olam.