How do they collect these stats?
I remember years ago it was just done by measuring the OS version of those visiting the App store, thereby excluding huge numbers of casual users.
These numbers seem suspiciously high. I know lots of people who follow tech closely upgrade quickly but there are tens of millions out there who have a 6, 6S, 7, SE on iOS 10 who won't be rushing to upgrade and probably aren't even aware there is an iOS 11.
They are high because they are usage not installation stats.
Slight correction: The 47% statistic is iOS 11 usage not installation. Mixpanel's data is from people people who use their iPhone to visit sites they monitor, Apple knows the real installation number, that would be lower than 47% due to people who don't use their phones as much, so less likely to be counted in the Mixpanel's data and less likely to have updated to iOS 11
Compare Mixpanel's Android usage data to Google's installation data