He is considered legendary due to the reasons that the Chinese character 'Tuna,' the common character used in the Sano family's real name, was not included in Genzaemon's real name, the Chinese character 'Gen' was included in his name despite his main family name of Fujiwara and his home town was Shimotsuke Province, not Kozuke Province.
だが佐野氏の通し字は「綱」であるが源左衛門の諱にはその字が入っていないこと、本姓が藤原氏にもかかわらず通称に源の字が入っていること、佐野氏の本領は上野国ではなく下野国であることなどから伝説の人物とされる。