One tribe changes hands. Levi is handed to Aaron as netunim netunim — given, given — as a substitution for every firstborn in Israel, because Egypt’s firstborn died and Israel’s did not. An exchange has to balance, so both sides are counted: 22,000 Levites against 22,273 firstborn, the surplus bought back at five shekels each. One sum does not balance. The Levite clans add to 22,300, and verse 39 says 22,000.