Jamieson, Fausset and Brown on Joshua 21:41

41. All the cities of the Levites within the possession of the children of Israel were forty and eight cities with their suburbs--This may appear too great a proportion compared with those of the other tribes. But it must be borne in mind that the list given here contains the names of every Levitical city (see on 1Ch 6:39-66); whereas only those cities of the other tribes are mentioned which lay on the frontier or along the boundary line. Besides, the Levites were not the exclusive inhabitants of those forty-eight cities; for there must have been also a considerable number of people kept there to cultivate the glebe lands and tend the cattle. Still further, the Levitical cities had nothing but "their suburbs round about them" [Jos 21:42]; whereas the other cities in Israel possessed a group of independent villages (see Jos 17:1-19:51).