What is the meaning of "Nichalu"?
Rashi: It is like Hinchilu (they caused them to inherit).
Why does it say "Roshei ha'Avos l'Matos"?
Radak: It is like Avos ha'Matos, i.e. the heads of the Batei Avos of each Shevet.
Radak citing Gitin 36b: The Torah equates leaders to fathers. Just like fathers can bequeath as they want to their children, also leaders can bequeath as they want to the nation (Hefker Beis Din Hefker).