Comey, in his testimony, will also say that he had informed the president that he was not personally under investigation. But the former FBI director refused to publicly state this because it would “create a duty to correct, should that change.”

Apply that to the Hillary announcement. This would explain his reluctance to address the ongoing investigation early on because of the "duty to correct". I'd like to see his notes about the pressure brought to bear getting him on the record while that investigation was underway.