Since it's possible for someone to get the coronavirus and not show any symptoms, this someone would never be tested. Therefore, even though they had the virus they would not "count" in the calculations
Yes, although unfortunately all we to compare with is other flu viruses that are most likely to also be under similar stats of people who were not tested. I know there have been times when I've seen people in the family be sick of the same flu disease and one might be sick for 2 weeks and another 24-48 hours and back on their feet. Personally. I've have little doubt that I've had some pretty bad flu bugs but never got tested for anything.