The sons of Joseph became two tribes. Thus it seems like Joseph received a double blessing of inheritance in the land (see Genesis 48:1–22; Ezekiel 47:13). Presumably the descendants of Manasseh and Ephraim were also very populous since they received large portions of land and the land was divided to correspond with population (Numbers 26:52–56). Why this happened with the tribe of Joseph rather than another is not clear; though it may have something to do with Joseph’s exploits in Egypt on account of which he was blessed with fruitfulness (Genesis 49:22) and honoured as a prince among his brothers (Genesis 49:26).
4 For the people of Joseph were two tribes, Manasseh and Ephraim. And no portion was given to the Levites in the land, but only cities to dwell in, with their pasturelands for their livestock and their substance.