[TonyCoffman]

This is an artifact of how the Japanese system works. In a nutshell, they track households (families) with individuals as sub records of the family record.

Everybody on the family register record shares the same surname. Non-citizens are listed as distrinct references (for foreign spouses and the like) and they may have a different surname from the household record.

There is more to it than this but that is the key thing to know about the system.