I think Windows Server has a large market share in medium size businesses. Note: they didn't specify "web" server. They didn't specify much. Who knows, maybe they count google as one server?
Also, I sometimes think the experiences of us here may be out of touch with what most ordinary businesses do.
I couldn't see linux units mentioned there, nor their methodology, or definition of "server".
EDIT2 The bottom of the press release: includes quarterly shipments (both ISS and upgrades) and revenues (both customer and factory), segmented by vendor, family, model, region, operating system, price band, CPU type, and architecture. Doesn't really help, but there's also a phone number and email address.
These numbers almost assuredly are based on "units shipped" which means you only count a Linux sale if someone orders a server from Dell or HP and requests a RH or Suse Linux license with it, rather than a Windows Server 2008 license or the default.
Because, as others have mentioned, the vast majority of large webhosts or Linux users "roll their own free distro" or purchase an enterprise agreement from RH or Suse, they will never be counted.
Microsoft always plays games with the numbers. They are playing a similar game with Windows 7 numbers - You can buy a Windows 7 upgrade for $99, which only gives you a single license, or you can buy a "family upgrade pack" for $99, which gives you 3 upgrade licenses.
Guess which one allows them to count 3x the number of licenses sold? Right, and they made it so that the family pack can even upgrade pirated versions of XP (I know this because I used this to bring one of my home copies up to legit status). It's a genius marketing move because for basically $33 per computer they get all the pirate computers to become fully legit, counted installs of Windows.
Never mind all the double-counting and triple-counting of licenses - MS has always been king at this. They force Dell/HP to bundle a license of Windows 7 with every computer sold knowing that corporations have Volume License Keys and wipe and reinstall Windows with their corp version. 2 copies of Windows sold for every physical computer.
What I would really like to see are the numbers of Windows computers receiving updates. That would be a legitimate number.
> I sometimes think the experiences of us here may be out of touch with what most ordinary businesses do
Not necessarily ... at least not in Europe where I live ... if you're a consultant that gets hired, try explaining to a client that they need to also buy a Windows Server license (or several); or pay extra per instance (in the case of cloud computing like EC2).
EDIT funfacts: Here's their source for "linux servers, 21.2%" http://blogs.computerworld.com/15675/idc_windows_dominates_l... which quotes: http://www.zdnet.com/blog/microsoft/behind-the-idc-data-wind... which in turn quotes: http://www.zdnet.com/blog/btl/server-sales-show-signs-of-lif... which quotes this IDC press release: http://www.idc.com/getdoc.jsp?sessionId=&containerId=prU... and here's IDC's description of their information product: http://www.idc.com/getdoc.jsp?containerId=IDC_P348
I couldn't see linux units mentioned there, nor their methodology, or definition of "server".
EDIT2 The bottom of the press release: includes quarterly shipments (both ISS and upgrades) and revenues (both customer and factory), segmented by vendor, family, model, region, operating system, price band, CPU type, and architecture. Doesn't really help, but there's also a phone number and email address.