Not the first time for Windows, Windows 7 is v6.1 internally, then Windows 8 and 8.1 get 6.2 and 6.3[1]. Main explanation is basically "programmers suck at checking version numbers" so Microsoft tried to avoid bumping the major version. I guess they risked pushing the version number to 10 for the "last" version of Windows, but have fallen back to just not touching it now that a new generation of management have decided that it isn't the last version of Windows.

[1] Windows 95 also tells 16 bit apps that it's v3.95, but 32 bit apps get the correct 4.0.