A very specific scenario in which every person on board was exposed to the virus and repeatedly tested.   
Your logic only works if 100% of cases are known and recorded, that is clearly not happening, therefore the numbers are flawed. You can’t use all cases that have run their course when you don’t know of all cases, as I already pointed out over 50% of cases on board the diamond princess they did not know they even had it as they showed no symptoms.