I'd hazard a guess that DistroWatch is doing a bit of user agent string translation. The table below lists platform tokens as reported by various types of Windows.
platform token -- Windows edition
Windows NT 6.1 -- Windows 7; Windows Server 2008 R2
Windows NT 6.0 -- Windows Vista; Windows Server 2008
Windows NT 5.2 -- Windows XP x64 Edition; Windows Server 2003
Windows NT 5.1 -- Windows XP
Windows NT 5.01 -- Windows 2000 with Service Pack 1 (SP1)
Windows NT 5.0 -- Windows 2000
Windows NT 4.0 -- Windows NT 4.0
Windows 98; Win 9x 4.90 -- Windows Millennium Edition (Windows Me)
Windows 98 -- Windows 98
Windows 95 -- Windows 95
Windows CE -- Windows CE
What DistroWatch calls "Windows 2008" is in all likelihood Windows 7.
platform token -- Windows edition
Windows NT 6.1 -- Windows 7; Windows Server 2008 R2
Windows NT 6.0 -- Windows Vista; Windows Server 2008
Windows NT 5.2 -- Windows XP x64 Edition; Windows Server 2003
Windows NT 5.1 -- Windows XP
Windows NT 5.01 -- Windows 2000 with Service Pack 1 (SP1)
Windows NT 5.0 -- Windows 2000
Windows NT 4.0 -- Windows NT 4.0
Windows 98; Win 9x 4.90 -- Windows Millennium Edition (Windows Me)
Windows 98 -- Windows 98
Windows 95 -- Windows 95
Windows CE -- Windows CE
What DistroWatch calls "Windows 2008" is in all likelihood Windows 7.
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