Not sure why they wouldn't be, it would appear that they still have what would be considered total control over the OS market. Also you could project that Windows 10 in the future will be the OS of choice. . .:)

The changes in the 8 & 8.1 numbers are likely due to retailers dumping old hardware inventory loaded with 8 at fire sale prices. People are jumping all over it, knowing that an upgrade is free. Machines with 8 are substantially cheaper than 8.1 machines, even with all else being equal. Certainly some deals out there!

And then, on top of that, many are trying to wait a few weeks or months for the upgrade, so that MS can hopefully get more of the bugs worked out.
Just My $.02,
Gene

Vista has dropped a point or two to fall behind Linux, which has incrementally gained a little, as has OSX 10.10, to the slight detriment of Windows overall.
No doubt Windows 10 had gained a few converts and lost some of their presbyterian users, who find the upgrade tactics, and perceived hype unbearable.

Some will have made the upgrade and then reverted in horror, especially if their hardware threw a wobbly with Windows 10

I doubt if MS is overly worried, but knows that Windows 10 Redstone must satisfy the critics within the Windows camp, otherwise the free upgrade year will have been in vain, just allowing a portion of Windows users to legitimize themselves, with the die hards digging in their heels even further.

Be nice to see them graph total recorded seats by month instead of market share.