None of those graph ever are. Because Linux users are more prone to tinkering, keeping stuff private and blocking a lot of bullshit scripting, there is big dark area of Linux systems listing themselves as something else (usually Windows because of those asshole websites not playing well with Linux by design without an actual valid reason other than "we don't like Linux").

(PS: For the same reason a lot of sites believe I'm using Chrome. I don't but I know they like to fuck with me when they see that Firefox user agent.)