The numbers at the top are misleading. Are they the average over the graph's time interval? Average over June? Not clear, but I suspect the latter.

If you switch the graph to daily, you see that it passed 51% on June 10th, i.e. in four weeks, and it's at 66% now.

http://gs.statcounter.com/ios-version-market-share/mobile-ta...

But yeah, the big question is how stubborn that long tail is, in the face of news reports about serious bugs or not.