Adam Clarke on Numbers 3:16

Moses numbered them - Though Moses and Aaron conjointly numbered the twelve tribes, yet Moses alone numbered the Levites; "for as the money with which the first-born of Israel, who exceeded the number of Levites, were redeemed, was to be paid to Aaron and his sons, Num 3:48, it was decent that he, whose advantage it was that the number of the first-born of Israel should exceed, should not be authorized to take that number himself." - Dodd, from Bishop Kidder.