User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4902593-20121229113429/@comment-5774152-20121230211450

Well in The Son of Neptune, it is mentioned by one of the giants(can't remember which one) that they would've attacked years ago right after the civil war ended but that it didn't matter because the world was almost as bad of shape now anyway. So maybe the prophecy said it needed seven demigods because the world is so messed up now that they need more people to help bring peace again.