It peaked at 4.05% in March. The last 2 months it went just below 4% as the Unknown category increased. For June the reverse happened, so 4.04% seems to be the real current share of Linux on Desktop as desktop clients were read properly/werent spoofed.
They get the data from user-agent strings, so it may not be 100% accurate
User agent strings are frozen these days, at least in Chrome. They still have the browser major version and OS name at least, but Windows will always report Windows 10, Android will always report Android 10, MacOS will always report 10.15.7, and Linux is just “Linux x86_64”: https://www.chromium.org/updates/ua-reduction/
User agent strings are essentially deprecated and nobody should be using them any more. They’ve been replaced by User-Agent Client Hints, where the site can request the data it needs, and some high-entropy things (ie fields that vary a lot between users) can prompt the user for permission to share them first.
Oh nice. Googie once again deciding for the entire Internet what it should be using and forcing it down everyone’s throats.