In a rural area I lived in in Japan there was an abundance of Chibas and Sugawaras. I asked if they were all related. I was told that some were, but mostly not: since family names were reserved for the samurai class during the Tokugawa period, some families lost touch with their ancestry and didn’t have names going into the Meiji period; so, they took the name of prominent families that had names and with whom they usually had some kind of subordinate relationship. Or, that is what I was told.