The way I read this graph is that 100 years ago, we averaged 10 more inches of snow a year than we do now. Or put another way, average snow decreased by 45% over 100 years.
To be fair, the downtrend started way early - around the 1910s to 1920s. By the 40s and 50s, we were getting similar amounts as now. Then the 60s went bonkers, then back down to around 13-15” a year.
I wonder though, is there a discontinuity in this graph when they changed the measuring station to DCA? And what year was that?