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've had so many issues with Ubuntu in the last few years compared to other distros that honestly I wouldn't feel comfortable recommending it as a beginner distro anymore.
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.
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.