Why did Hiruzen refer to Yamato by the name Yamato?

[oShux]

Think about it. Yamato's real name is unknown. Infact all we know about him is his code names. When he was in the foundation it was kinoe, when he was in anbu it was tenzo, and for the purposes of joining team kakashi he named himself Yamato. And he joined team Kakashi after Hiruzen died.