A. The primary reason is application compatibility. A main goal for Windows 7 was to not break application compatibility.

Many applications break if you change the major version number, but fewer break if you change only the minor version, so it was decided to improve application compatibility by keeping the OS version 6, the same as Windows Vista.

You can think of Windows 7 as version 7 in terms of functionality, but 6.1 for API. Hopefully, in the future applications will check if functions are available rather than just checking for an OS version.

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