The scale on the left doesn’t start at zero, so the difference is smaller than the size of the bars make it seem. The difference between #1 Slackware, and last spot Arch, is 0.75 points in a 0 to 10 scale, but the bar size of Slackware is about 2.5x bigger than the bar for the Arch users.
I think this is it.
The historians I know of actually seem to lean quite left of the average person; it’s the light hobbieists, who are often more interested in the aesthetics/surface stuff, who seem to fall victim to the alt-right stuff.