And why is Arch more popular than Ubuntu? Surely SteamOS counts as something different, so it's probably not that.

I'm not a fan of Ubuntu, but it's a very popular beginner OS, and I'd assume a lot of Linux gamers are lazy and use the thing that gets them into a game the fastest.

I use "SteamOS Holo" btw

Why are Arch and Manjaro in quotes, but Ubuntu LTS and Linux Mint aren’t?

They're probably putting the rolling releases in quotation marks -- I'm guessing they're pulling the Description field from "lsb_release -a", where "Arch Linux" says just that, while each Ubuntu/Debian/Mint/etc distro will show specific version numbers (and that would explain why Arch shows up as a higher share than Ubuntu 22.04.3 LTS) -- I'm sure there are several more Ubuntu entries in their list that would total more than Arch's percentage. I'm not sure why they arbitrarily truncated the Linux list at 4 while showing 5 Windows/Mac releases, though.

EDIT: Found another screenshot where they list "SteamOS Holo" in quotes, too. So I guess they just include quotes for every distro that doesn't show a version number in that field.

Version numbers I'm guessing.

Do they not have actual version numbers maybe?

Which means, Ubuntu may have several separate entries, whereas Arch gets all combined altogether. If that's the case, then likely not a very accurate Linux distro list without additional data cleaning to combine versions of distros.