In Y'hoshua 15:15, and again in Shof'tim 1:11, going to the city of דְּבִיר is referred to as "going to יוֹשְׁבֵי דְּבִיר" (something if not unique then nearly so (in Y'hoshua, anyway; I haven't learned much of Shof'tim recently)). Why?
There are some other places where יושבי is used similarly. In Josh. 17:7 ישבי עין תפוח is used as a geographical marker, and in v. 11 there seems to be a distinction between the cities and their inhabitants: בית שאן... ויבלעם... ישבי דאר... וישבי עין דר... וישבי תענך... וישבי מגדו. But that just strengthens the question as to what this difference means.
It's possible that יושבי פלוני means "the people who resettled the city after it was abandoned (or destroyed) previously." We do find the kings of the "יושבי" cities listed among the 31 kings that Yehoshua defeated (Josh. 12:9ff): דביר, תפוח, דאר, תענך, מגדו. Whereas by contrast, בית שאן and יבלעם are indeed not listed there. So in other words, "going to יושבי דביר" would mean trying to dislodge the new inhabitants.
The problem I see with this idea is that it doesn't seem to fit the information given in Judges. In 1:27, where the list of Menashe's unconquered cities is repeated, תענך and יבלעם are switched around in position and in the presence or absence of ישבי. Also in vv. 31 and 33 several cities are mentioned with ישבי attached, but those are not among the earlier conquests.