As the latter especially is called "Zoku sandai jitsuroku," the commonly accepted theory holds that it was called so ever since Nobutomo BAN, because after the 40 volumes written about the reigns of the two Emperors Uda and Daigo, 10 volumes were added on the reign of Emperor Suzaku.
後者を特に『続三代実録』と呼称されている事から、伴信友以来、宇多・醍醐の2代が書かれたのが40巻で残りの10巻で朱雀天皇1代が追加されたために『続三代実録』と呼ばれたとする考え方が通説とされている。