Near the beginning of the Book of Numbers (from roughly 3:39 to 3:51), G-d commands that all the firstborn Israelite males need to be redeemed via a 1:1 'trade' with a Levite. There were 22,000 eligible Levites, with the remaining 273 Israelite firstborns redeeming themselves via a monetary payment.
Rashi on verse 3:39 questions why weren't there enough Levites, when the counts from the individual Levite families totaled 22,300 Levites. He answers (based on a Gemara in Bechoros) that there were 300 Levites who themselves were firstborn, and therefore inelligible to partake in the redemption of the Israelite firstborn, since they were 'redeeming themselves'.
While the narrative is straightforward, what strikes me as interesting is the ratio between firstborn Levites and 'standard' Levites. With 22,300 Levites and 300 of them being firstborn, that comes out to less than 1 in 70 Levites being a firstborn. That seems like an unusually low number of firstborns to me (or on the flip side, an unusually high number of non-firstborns)! Do any of the commentators address this seemingly unusual ratio?