Most likely because their "C++ developers by platform" statistic does not include an "other" category, while there are actually developers working on platforms than Windows, OS X, or Linux (such as Solaris, BSD, etc). Since they didn't include an "other" category but the numbers add up to 100%, they probably just left out people who didn't respond to that question; thus in fact, in their sample, the number of Windows developers is probably less than 36%.
Another thing to note is the sources of information used for these statistics, including their own user surveys. These aren't necessarily accurate for the industry as a whole, but rather represent a particular sample, which happens to include their own existing users, people on Reddit, people sharing code on GitHub, etc.
Another thing to note is the sources of information used for these statistics, including their own user surveys. These aren't necessarily accurate for the industry as a whole, but rather represent a particular sample, which happens to include their own existing users, people on Reddit, people sharing code on GitHub, etc.