Number of Levites

In yesterday's reading, Numbers 3:39 lists the number of Levites as 22,000. But if you add up 7,500 Gershonites, 8,600 Kohathites, and 6,200 Merarites, you get 22,300. The discrepancy is either a copyist error (see footnote on verse 28) or, according to Kretzmann, Jews say that 300 were dropped from the total because "they were firstborn so they could not be used in comparison with the other tribes."