Exodus 29:26 - Matthew Poole's English Annotations on the Holy Bible

To wit, the breast alone, whereas both shoulder and breast were given to Aaron afterwards; the reason whereof might be, either because Moses was not a proper and complete priest, as Aaron afterward was, but only appointed by God for this time to do that work; or because now there were in a manner two priests, the one consecrating, to wit, Moses; the other consecrated, to wit, Aaron; therefore these parts were divided, the breast went to the former to be eaten, the shoulder offered unto God for the latter, Exodus 29:22; he being not yet a perfect priest, and therefore not in a capacity of eating it.