That all makes sense if they were only determining the OS version for internal purposes, in order to filter the content as you said (still, it's great to let us

That all makes sense if they were only determining the OS version for internal purposes, in order to filter the content as you said (still, it's great to let users know about it, otherwise people may get confused as to why they see different content on different machines/browsers). Since this is a public page, with the title "Which Windows operating system am I running?", I think the right thing to show when the user is not running Windows is, well, "You are not running Windows". That would clearly inform users, who may not be aware, of the fact that their OS is not Windows (and that, most likely, that website will have nothing to help them with using their current OS). Another alternative would be "Can't determine Windows version". Anyway, they way things are now, that page is definitely wrong.