"We'll make it nine," Sirius said decisively, before explaining his reasoning. "He's six, so nine is... When you turn a six around." He demonstrated with his hands. Might not be a conclusive reasoning, but at least he had a reasoning. "So, you should really be asleep, Harry. Wanna go get ready and then Remus can watch how I tuck you in?"

He wanted to include Remus in their routine, such as it was, as quickly as possible.