“ For the Levites have no portion among you, for the priesthood of YHWH is their inheritance, and Gad and Reuben and the half tribe of Manasseh have received their inheritance Beyond Jordan Eastward, which Moses the Servant of Yahweh gave them.”
The explanation is given as to why there were only seven portions required. It was because Levi had a special inheritance. Their inheritance was the priesthood and serving the Tabernacle. Their portion was YHWH (Joshua 14:33). This has been constantly stressed. To partake in the service of God is the greatest possession a man can have, and in order to enjoy it he should put aside all earthly possessions and have his whole heart fixed on God and His service. It should be noted that the Levites were only provided with the minimum required to enable them to survive.
The other factor involved was that an inheritance had been given to Reuben, Gad and half Manasseh in Transjordan.