The simplest explanation is that describing a disease as more or less fatal (in this context) only answers the question, “if you already have the disease how likely is it to kill you?”
The total number of fatalities caused by a disease on the other hand depends on the spread across the population which is influenced both by the communicability and by the effect of the symptoms on its spread—how long are people infected, how long do the infected live, how identifiable are the infected.
The total number of fatalities caused by a disease on the other hand depends on the spread across the population which is influenced both by the communicability and by the effect of the symptoms on its spread—how long are people infected, how long do the infected live, how identifiable are the infected.