Sforno on Numbers 1:53/ספורנו על במדבר א׳:נ״ג

1והלוים יחנו סביבת, this is another difference between the Levites and the other tribes, i.e. that they were to be encamped immediately adjacent to the sacred domain of the Tabernacle. Whereas the other tribes encamped each around its flag, the Levites’ “flag” was the Tabernacle.