Can we really trust such data? How good is it statistically? I mean by looking at User-Agent string of web page requests you can try to guess what OS made it, b

Can we really trust such data? How good is it statistically? I mean by looking at User-Agent string of web page requests you can try to guess what OS made it, but it is just about the web requests and an arbitrary string that the user can change – can you figure the real OS market share just based on that? They even say they cover only 1.5 mil websites but for sure there are magnitudes more hosted.