Netmarketshare also has Windows XP at 13.57 per cent, then 18.26 percent before a rise to 18.93 per cent, a step up for which your correspondent can't make a convincing argument.
Well, the easiest answer is probably correct: it's shit data. A slightly more nuanced data would be that some of the sites where Netmarketshare is used are seeing visitors move from desktop to mobile. This will lead to a shift in market share in the desktop that cannot be explained by looking at those numbers alone. Incomplete data is, of course, shit data.
As El Reg never controls the reports with its own data these articles only benefit is letting us commentards point out the elementary errors in them.