Because in 1918 we didn’t include homicides, heart attacks, diabetes, cancer as flu deaths.
Don’t forget getting hit by trucks or running ones motorcycle into a telephone pole.
The U.S. in 1918 had a population of about 105,000,000 out of that perished 675,000 from Spanish flu. There are about 3 times as many people here now and about 2/3 the deaths. That works out to about 20% of the impact of the Spanish flu. There are lies, damned lies, and statistics.