Musar zu Dewarim 18:2

Clearly by dividing the letters of the number twelve in this fashion Rabbi Nechemiah wanted to convey something. We find at least one occasion when the twelve tribes are divided into a group of seven and into a group of five. In Parshat Shoftim (Deut. 18,2), when discussing the word נחלה, inheritance, Rashi quotes a Midrash by a Rabbi Klonimos who understands the words: ונחלה לא יהיה לו, "he will not have an inheritance," as referring to five tribes, whereas the next two words in the verse: בקרב אחיו, "among his brothers," are understood to apply to seven tribes. The "five" tribes are those to whom Moses allocated territory, whereas the seven tribes are those who had their territory allocated by Joshua.