Why do Xiongxian and Bazhou belong to different geographical administrative regions and have almost the same accent?

The accents of Bazhou and Xiongxian are exactly the same, but apart from the accents of these two cities, the accents of other areas in the middle of these two counties are different. I think it may be because the people in these two places are the earliest locals, and people from other places immigrated from other places. The most prosperous period of Bazhou and Xiongzhou was the Song Dynasty. In the Ming Dynasty, a large number of Hebei immigrants migrated from Shanxi, and later there were other immigrants. In fact, this generation of accents has a dividing line.