Re: Why is Win 8 and Win 8.1 seperated?
8.0 and 8.1 arguably should be counted together for the purposes of the graphs here. It's a free update, and aside from the update method, it's not really very different from some of the relatively large changes made in service packs for prior Windows OSes.
HOWEVER, it should also be noted that it's extremely unlikely that both added together would reach anywhere remotely near the actual install base of active Windows XP systems. Remember that these numbers are derived from Internet stats based off trackers on certain public websites, so they're not going to include systems only accessible from internal networks, or those that see little to no web use. Additionally, the stats by Statcounter are based off total page views rather than unique visitors, so systems that see the most web browsing use, which tend to be consumer systems, will end up greatly overrepresented. Statcounter is also only used on around 2.5% of web sites, so it can't see the vast majority of the web. Meanwhile, Net Applications is installed on far fewer sites still, and it doesn't release much information on how it comes to its numbers, though they seem to be based more on estimates of unique systems rather than page views.
Neither web-tracking service is likely to be a particularly accurate way to measure the relative install-base of various operating systems though. They undoubtedly overrepresent certain geographic regions and demographics of users, while underrepresenting others. The best you can say is that within the sample obtained, these are the results they came to, but without knowing exactly what that sample consists of or what those methods are, the results are of limited use.
If you know the sampling method and demographic on the other hand, the results can be a lot more meaningful. For example, Steam's Hardware and Software Survey likely provides a reasonably good representation of systems in use by the core PC gaming market. In that case, approximately 60% are on Windows 7, 28% on Windows 8/8.1, 5% on Windows XP and 2.5% on Vista. Additionally there's around 3% on OSX and 1% on Linux. This is useful information for someone looking to develop a game or piece of gaming hardware. Of course, it's not at all representative of the wider operating system install base, since it's almost entirely based on data from relatively new consumer systems used for gaming. Web trackers like Statcounter and Net Applications target a more diverse audience, but you still need to remember that they undoubtedly misrepresent worldwide systems as a whole by a pretty wide margin, and it's extremely difficult to determine just what demographics they do cover.