Why does it say "l'Mishpechos ha'Kehasi"? It does not say so for Gershon or Merari!
Bi'ur ha'Gra: Kehas has two families - Kehunah and Leviyah.
How did this lottery work?
Malbim: The Leviyim were split into four groups - Kohanim, the rest of Kehas, Gershon and Merari. Each received among a set of three Shevatim 1 (whose stones on the Choshen lit up when that group's lot came up). All received what was proper for them; the more Kadosh parts of Levi received closer to the Mikdash; the Kohanim received among Yehudah, Shimon and Binyamin. 2
Chomas Anach, citing Kli Yakar (Lani'ado): First the lottery said to give to the Kohanim amidst Yehudah (who is king), then to Shimon, who is amidst Yehudah? It was miraculous Hashgachah, that the most Kadosh received the most Kadosh parts!
Malbim: Next was the rest of Kehas, who carried the Aron and most Kadosh matters; they received among Yosef (which was next to Binyamin, and a strip from Yosef extended to Shilo) and Dan. Gershon received from Shevatim next to Yosef (Chetzi Menasheh's land in Bashan in Ever ha'Yarden went with Naftoli, who received the Bashan in Eretz Yisrael proper) and Merari received in Ever ha'Yarden and Zevulun.
