Statcounter are focused on web traffic stats... so the most likely thing is that those analytics are considering "only" devices used by people visiting webpages.

So, maybe the title of this chart should mention that... something like "OS of devices used to view web pages".

If this is correct, then it probably there are more users using mobile devices than the counted in this analysis, because a lot of people use the smartphone never to visit web pages, but they only use apps (Facebook, YouTube,. blah blah...). 😁