A quirk of statistics. The most severe cases are seeking medical care, getting tested, and confirmed. The open end on the numbers comes from not knowing how many mild cases there are out there who were not tested and not confirmed.
Without knowing that, (because people with a mild case may not seek medical attention at all), the numbers will make the disease look more deadly, and less widespread at the same time.
Keep in mind that there are more people killed per day in car accidents in the US than this desease had killed worldwide.
Excellent point about the mild cases never being on the radar, and therefore never counted in the stats.