The concentration of surnames in certain areas in China is the result of "in-breeding", that phenomenon had stretched hundreds or even thousands of years.
In the ancient past, the Chinese were clustered in small villages. Due to lacking means for transportation, the villages tended to be isolated from the outside world, to which the villagers had poor knowledge about but suspicious and distrust.
The village usually started quite small, maybe a few families totaling a few hundred people. The men tended to stay in the village either for inheriting the establishment of their ancestors or to maintain the line of work that had carried through generation to generation. They usually marrying the neighboring woman, who lived in the same village, and the elders of the two families tended to be close friends.
Similarly, the women were tended to stay in the village forever, as the parents were worried about their daughter might be mistreated, and hopelessly abused. It was a prevalent practice that the rich family will find a poor young fellow in the village to marrying into the bride family with the condition that their child ought to be using the bride's family's surname. For young women from poor families, they often became the collection (second wives) of the local rich old men.
As the phenomenon continued for centuries, there was enormous population growth but the number of the families remained constant, so did the family names (surname). And as the family roots got stronger and deeper, people tended to be homebound rather than leaving, thus the clustering of a few family names around an area was quite common until WWII, and later the Cultural Revolution triggered by Mao, the bond was then broken.
Finally, it is worth noting that many poor families were sold to the rich family as servants, and for the convenience of remembering everybody, the host family would usually assign a new name to their servants, as well as giving them the host surname. It was for the bragging right of the host, and to distinguish to whom the servants belonged. inter-marriage between servants was commonplace, and their child would marry another servant's child and give birth to more children... the most notable thing is, after all, they all have the same family name (surname).
Hope this makes sense to you.