Numbers 3:14,15 - Coke's Commentary on the Holy Bible

And the Lord spake unto Moses— Bishop Kidder remarks here, very judiciously, that though the number of the Israelites was to be taken by Aaron as well as by Moses (chap. Numbers 1:3.); the precept to number the Levites is directed to, and executed only by Moses, Numbers 3:16. In like manner we find that Moses only was concerned in numbering the first-born of Israel, Numbers 3:40-42. 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, Numbers 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 authorised to take that number himself. We may observe too, that the Levites being numbered to know how many there were in all, (not how many were able to bear arms, chap. Numbers 1:3.) that so there might be an exchange of them for an equal number of the first-born; it was proper that they should he all numbered without distinction; even from a month old; the age at which the firstling males were appointed to be redeemed.