It is because while "Nihonshoki" says that the Emperor Tenchi was born in 626, "Ichidai Yoki" (summary chronicle of each reign) and "Honcho koin jounroku" (the Emperor's family tree, made in the Muromachi period) says that the Emperor Tenmu was born in 622 and "Konendai ryakki" (Short Imperial Chronicle) says he was born in 623 which is a few years earlier than that of the Emperor Tenchi.
鎌倉時代に成立した『一代要記』や『本朝皇胤紹運録』、『皇年代略記』ではそれぞれ生年が推古天皇30年（622年）・31年（623年）と考えられ『日本書紀』での天智天皇の生年・推古天皇34年（626年）を上回る事等がその根拠とされている。