I was about to comment that SteamOS is based on Arch, but after looking at the actual graphs, they've got SteamOS as its own separate category.

I wonder how much of that is "hackers and experimentalists", versus random gamers* preferring Arch Linux's bleeding-edge latest-and-greatest packaging approach versus Ubuntu's seemingly-slower-paced development?

* though I suspect even the most casual 25% of PC gamers are probably significantly more tech-savvy than the average PC user of the population in general.