I've been saying this for YEARS and it's only become more and more significant.
Market share is merely the amount of computers that are sold that ship with a specific OS. But the actual number of people running computers with a particular OS on it (aka installed userbase) is a TOTALLY different number.
There are two primary reasons between the difference in the number of Windows vs Mac computers sold (marketshare) and the number of Windows vs Mac computers that people actually use (installed user base).
1) Macintosh computers are used for much longer periods before being replaced be a new computer.
For instance, if 10 people buy Macs and keep them for 2 years before they buy new replacement Macs, and 10 other people buy Windows machines but buy new replacement Windows machines after 1 year, then the market share would show 10 Windows machines bought every year but only 5 Mac macs bought every year, which suggests that there are twice as many Windows machines out there - but in reality there are 10 windows machines out there being used but also 10 Macs out there being used.
2) TONS of machines that get sold with MS Windows on it are absolutely irrelevant to the number of people actually using that OS on their primary computer.
SO many of the computers that get sold with MS Windows on it wind up being used in ways that are meaningless, such as in ATM machines, kiosks, etc. And TONS and TONS (and TONS!) of computers sold with MS Windows on are used in a manner where the OS is basically meaningless, meaning the user can't install any applications, they can only run the small number of basic programs allowed by the system administrator. This includes ALL the computers that are used in ways where the user is only given basic system privileges, such as ALL the MILLIONS of machines that are in the cubicles of businesses with managed computers, all the computers in libraries, school rooms, college computer floors, etc.
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Once you subtract the number of computers where the OS is meaningless and offset the length of time that a Mac vs an MS Windows computer will be used before being replaced, then you wind up with a number much closer to accurately representing the number of computers that are ACTUALLY running a particular OS in a meaningful way.
Once you factor out those aspects you get close to actual number of computers that people have installed on the computer in their real life.
And so the number of computers running OS X in a meaningful way vs. MS Windows shows a LOT more people using OS X than gets reported in the media.