Recently, Mike Nash indicated that the Windows 7 moniker would have no impact over the actual version of the Windows code. In this context, Windows 7 turns out not to be Windows 7 at all, but actually Windows 6.1, the successor of Windows Vista which is v6.0. Steven Sinofsky, senior vice president, Windows and Windows Live Engineering Group, explained that Microsoft meant for Windows 7 to be Windows 6.1. 

“As we started Windows 7 we chose to keep the major version number at 6 so as to maximize compatibility for third party developers. This is really about our commitment to compatibility. And as you have seen with past releases, the major/minor nomenclature for the qualitative aspects of the release don’t necessarily line up with the numeric designations. What you’re seeing with Windows 7 is just a deliberate focus on compatibility over version number vanity (just as you saw with the “major” release of Windows XP),” stated Sinofsky.

Source:→ Softpedia