Charles Ellicott Commentary Numbers 3:39

"All that were numbered of the Levites, whom Moses and Aaron numbered at the commandment of Jehovah, by their families, all the males from a month old and upward, were twenty and two thousand." — Numbers 3:39 (ASV)

And Aaron ... — In the Hebrew text, the word Aaron has certain marks over it, known as puncta extraordinaria, denoting that it is to be regarded as spurious or doubtful. It is omitted in the Samaritan and Syriac versions and in a few manuscripts. There appears, however, to be no sufficient reason for its rejection from the text.

Twenty and two thousand. —The total of the three individual items—namely, 7,500, 8,600, and 6,200—amounts to 22,300. It appears, however, from Numbers 3:46 that the total is correctly given as 22,000, since the number of the firstborn, 22,273, exceeded that of the Levites by 273.

It has been suggested that in Numbers 3:28 we should read שלש (shalosh), three, for שׁש (shesh), six—that is, 8,300 instead of 8,600. Alternatively, if the numbers were denoted by letters of the alphabet, as has been commonly supposed, it is quite possible that one letter may have been substituted by the scribe for another.

Some suppose that the three hundred were themselves firstborn sons, born after the command to sanctify the firstborn, and that for this reason they were not included in the census. (See Bishop Wordsworth’s Notes in loc., where the reasons that may be assigned for the extreme paucity of this tribe, compared to the other tribes, are discussed.) The later census, which also included children from a month old and upwards, shows only a very small increase in the number of this tribe, the total on that occasion amounting to only 23,000 (Numbers 26:62).