Update: Mystery means "Pending"This post was originally titled "The Cheating Is Getting Worse", but based on what I learned since writing it, the title was misleading. While the analysis of "excess deaths" provides strong evidence that many COVID-19 related deaths are not reported as COVID-19 deaths in the death certificates, and that the officially reported numbers of COVID-19 deaths are significantly lower than the actual deaths, the classification of deaths in the "mystery" (R00-R99) category is not a sign of cheating; as a reader pointed out, this range includes the R99 category where the cause of death is "pending", with further investigation needed. Over time, most deaths initially classified in the R99 category are assigned a defined cause of deaths.
So the "mystery deaths" are mostly just temporary mysteries which in all likelihood will be solved over time. Some of these mystery deaths may indeed be caused by COVID-19, as the note at the bottom of this page illustrates; even if they are, they may not be classified as such if not positive COVID-19 test exists. However, some of the "mystery" death are likely to be due to other causes, for example accidents and drug overdoses, which can take many months to be resolved.
Details about how death certificate are defined by the state, and thus differ from state to state. Similarly, states differ on how and when they report death statistics to the CDC. Many states change most "Pending" R99 death certificates within a couple of months or so, but others, including Florida, may take up to a year to change them. This can be seen in data from before the COVID-19 epidemic, and has been previously described by the CDC.
While the death reporting patterns may be peculiar, and it may be hard to understand why states like Florida often need half a year or longer to determine a final cause of deaths, there is currently no evidence that indicates that the R99 category is used on purpose to hide COVID-19 related deaths.
The purpose of excess death analysis is to get a clear picture of the impact of a disease, regardless of any testing issues. It is not to support conspiracy theories based poorly understood or incomplete facts. I therefore deleted the original post. I plan to re-examine excess deaths and COVID-19 death reporting for different states in a future post.
Note added 6/28/2020: A look at excess deaths in different states, with a close look at Texas, is now available at https://covid19science.blogspot.com/2020/06/covid-19-deaths-closer-look.html