Malbim: In Yeri'as or Hashlachas Goral, he casts it from his hand without perceiving what is written on it, e.g. "Hipil Pur" (Esther 3:7). That is a chance Goral. Here, the lottery was based on Hashgachah; Hashem made it come up the proper way. It says Yipol - the casting is not attributed to man. Yeri'ah is throwing in an intended direction. This is the only lottery to which it applies ("v'Yarisi" - verse 6), for Yehoshua knew where each Shevet will inherit, based on the Brachos of Yakov and Moshe. Here he said va'Eshlachem, for it seemed that he does not know where it will fall.

Rashi: The seven portions should be according to the size of the seven Shevatim that still need to receive.

Malbim: Yehoshua knew that via Ru'ach ha'Kodesh, the Sheluchim will choose the portions that are destined for the remaining Shevatim.