https://www.nytimes.com/2020/04/05/us/coronavirus-deaths-undercount.html

Also https://www.washingtonpost.com/investigations/coronavirus-death-toll-americans-are-almost-certainly-dying-of-covid-19-but-being-left-out-of-the-official-count/2020/04/05/71d67982-747e-11ea-87da-77a8136c1a6d_story.html

Ok, then. As more tests indicate more cases, deaths go unreported, so the mortality rate appears lower than it really is.