That makes sense to me. The population has increased since 1980 but flu deaths remain constant, which means a lower mortality rate. Presumably there is some correlation between increased flu vaccination rate and decreased mortality rate.
It went from negligible to about 2/3 the population, but the population increased by about 50%. If the vaccine was 50% effective in preventing flu deaths, and there were no other changes (or if better care was exactly offset by increased vulnerability due to aging population) you would expect no change in total deaths.
Your intuition just looking at the multiplier on vaccinated population just seems to be a bad intuition.
> (or if better care was exactly offset by increased vulnerability due to aging population)
Why are you counting the old people twice?