When Y'hoshua 15:21–62 lists the cities of Y'huda, it does so by area/topography. M'tzudas David to 15:37 says that "everywhere it counts some separately and others separately", it does so by area or topography. I assume he's referring (at least) to 18:21–28, where the list of Binyamin's cities is divided into two but not explicitly according to any rule. Is my assumption right? If not (and even if so), is there any (further) explanation for the division in chapter 18? (Sources, please, if possible.)
To judge by Tevuos Haaretz's identifications of these cities (Hebrew text here, English translation here), it looks like the division is between the eastern and western areas of the territory. This also seems to be borne out by the map on this page at Wikiyeshiva, and at the bottom of the page they say so explicitly.