my best guess is that narutos mother died whiel giving birth to him. which also fits into the rest of my theory about narutos parents. the 4th was late to the fight with kyuubi, the reason to that was because his wife was giving birth to his son, naruto. she died while doing this so 4th decided to sacrifice his own life for the sake of the village so he decided to seal the kyuubi inside his son. no one would seal something strong as kyuubi inside a random child, and with sealing kyuubi inside his own son he knew it would be inside someone with strength enough to handle it once he finds out. and narutos mother was probably some no-name ninja from leaf.