The Allocation of the Other Cities.
‘ And the children of Gershon had by lot out of the families of the tribe of Issachar, and out of the tribe of Asher, and out of the tribe of Naphtali, and out of the half tribe of Manasseh in Bashan, thirteen cities.'
These Levites, who were ‘descended' from Gershon, were given responsibility for three northern tribes, including Transjordan Manasseh.
‘ The children of Merari, according to their families, had out of the tribe of Reuben, and out of the tribe of Gad, and out of the tribe of Zebulun, twelve cities.'
These were the ‘descendants' of Merari, the third son of Levi. They were given responsibility for the Transjordan tribes of Reuben and Gad, and for Zebulun.
Thus were determined and allocated between the Levites the forty eight cities promised by YHWH (Numbers 35:6-7).