They don't, actually. It isn't as if there are any official numbers, so there are just different estimates. I'm guessing you got yours from some M$ sponsored "study".
Could as well be. Then again, all links having statistics like that (including Wikipedia) show same results with very small error margin (then again, could as well be difference in having up to date monthly reports). So if we really assume it's all based on M$ sponsored "study", then there is no data that isn't "sponsored".
But, even Valve own H&S surveys shows that win7 is going away in the gaming world:
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(giving a link, because googling that may be an issue for some).
And honestly, knowing thing or two, I don't see a reason for M$ to bother with twinkering this type of data (other stuff, sure). They don't really have to. Why? Well, when majority of new computers are shipped with W10 and barely anyone bothers to install W7, usage of that certain OS will grow, not that complicated. Just like I can expect that majority of Apple computers are running Mac OS.
As much as I would want to go with conspiracy theories and full on rebellious perspective, I don't see why would they bother. Not when such companies as Google could easily prove that their data is far from reality (ofc, we can always assume that G$$gle is working with M$ and keep going this route every time - falling deeper and deeper into paranoia. Doesn't mean that those companies aren't shit tier when it comes to treating people as anything more than numbers, but let's not fall into madness.)