I'm kind of surprised to see that 13% of my visitors run Linux. It's actually a little less than that, since I also visit my blog (Quality Control - it's a never ending job) and I run Linux. For years, Linux has been hovering around mid-single-digit market share, so this is 2 or 3 times expected.
Same for Mac, at 14%. Apple has only recently approached 10% share of new systems sold, and overall installed base is likely no more than 7%.
I'm not sure why these OSs are so over-represented.