This was the list of the Levites according to their clans: of Gershon, the clan of the Gershonites; of Kohath, the clan of the Kohathites; of Merari, the clan of the Merarites.
57. And these are they that were numbered of the Levites. He treats separately of the tribe of Levi, which God had dissevered from the rest of the people; and of the sons of Levi, the last mentioned is Kohath, the founder of the sacerdotal family. Hence we may probably conjecture that the law of primogeniture was not regarded when God deigned to take the priests from thence.
Levi was God's tribe, a tribe that was to have no inheritance with the rest in the land of Canaan, and therefore was not numbered with the rest, but by itself; so it had been numbered in the beginning of this book at Mount Sinai, and therefore came not under the sentence passed upon all that were then numbered, that none of them should enter...
Commenting on Numbers 26:57-62
Num 26:57-59 Mustering of the Levites. - The enumeration of the different Levitical families into which the three leading families of Levi, that were founded by his three sons Gershon, Kohath, and Merari, were divided, is not complete, but is broken off in Num 26:58 after the notice of five different families, for the purpose of tracing once more the descent of Moses and Aaron...
Commenting on Numbers 26:57-59