Why is Liu Qianjin a descendant of Pengcheng family?

Are all differentiated from the Pengcheng family. According to the information of the hundred surnames, Liu Qianjin is a descendant of Pengcheng family. Judging from the source of Liu Qianjin's surname, they are all differentiated from Peng's surname, and they are the same clan as Peng's surname. Liu Qianjin and Peng often regard themselves as a family, and once had an agreement to share weal and woe and not to marry each other.