There are likely thousands if not hundreds of thousands of cases where people are infected, are never tested, recover, and go on about their lives. They probably just thought it was a cold or the flu, dealt with it like most people and moved on. For that one reason they have absolutely no idea what the actual mortality rate is for the Wuhan virus. They're basically just guessing at this point because they don't have a grasp on the actual number of cases out there.