1. Changing surnames: In ancient society, marriages and surnames between families were usually restricted. Cheng's surname may be the result of a family member in Lujiazhuang changing his surname during marriage or migration.
2. Adoption: In some cases, in order to inherit property or family relations, people will choose to adopt children. Cheng may be the surname of an adopted son of Lujiazhuang.
3. Concealing his identity: In the novel, Cheng is the real heir of Lujiazhuang, but in order to avoid being chased by his enemies, he concealed his identity and took "Cheng" as a pseudonym. Therefore, Lujiazhuang's daughter surnamed Cheng may be to cover up the real family relationship and identity.
It should be noted that the above is only an explanation of the plot and possibility in the novel, and the specific reason may be that only the author Jin Yong himself can give the exact answer.