Malbim: Also in Devarim (18:1,2), it says "Lo Yihyeh la'Kohanim ha'Leviyim? Chelek v'Nachalah? Isheh Hashem v'Nachalaso Yochelun; v'Nachalah Lo Yihyeh Lo b'Kerev Echav Hashem Hu Nachalaso." The first verse teaches that they do not receive an inheritance by themselves, like the other Shevatim. The latter teaches that even though they received cities, this was not "b'Kerev Echav" (each Shevet did not separate cities immediately after receiving its portion in the lottery). Rather, after all the Shevatim received, each designated cities, like Terumah, for Hashem Hu Nachalaso. This is why Moshe did not separate Arei Leviyim in the inheritance of Bnei Gad and Bnei Reuven (other than the three special Arei Miklat); Yehoshua separated them after the division was finished. The verses do not even specify what each of Gad and Reuven received, until saying (verse 14) that Levi did not receive a share.