Windows 7 needed to be Windows 6.1 because of all the software solutions, with an emphasis on setup programs, were tailored to a specific Windows version. In this context, an application or a driver that is hard coded for Windows Vista will fail miserably to install or run under Windows 7, if the operating system features version 7 for its code. Not the same is valid for Windows 6.1. Essentially, all software packages that support Windows Vista will automatically support Windows 7 (Windows 6.1).

Windows 1.x (dos)
Windows 2.x (dos)
Windows 3.x (dos)
Windows NT 3.x (NT)
Windows 4.x (Windows 95, 98 and ME)
Windows NT 4 (NT)
Windows 2000 (Windows NT 5.0) (NT)
Windows XP (Windows NT 5.1) (NT)
Windows sever 2003 (Windows NT 5.2) (NT)
Windows vista / server 2008 (Windows NT 6.0) (NT)
Windows 7 (Windows NT 6.1) (NT)

If you count it up, Windows 7 is the 7th NT code released