Ecco la spiegazione.
..”Anyway, the numbering we used is quite simple. The very first release of Windows was Windows 1.0, the second was Windows 2.0, the third Windows 3.0…
..”Windows 2000 code was 5.0 and then we shipped Windows XP as 5.1, even though it was a major release we didn’t’ want to change code version numbers to maximize application compatibility.
That brings us to Windows Vista, which is 6.0. So we see Windows 7 as our next logical significant release and 7th in the family of Windows releases.
..”Windows 7 is a significant and evolutionary advancement of the client operating system. It is in every way a major effort in design, engineering and innovation. The only thing to read into the code versioning is that we are absolutely committed to making sure application compatibility is optimized for our customers.”
Quindi è la scelta più logica, visto che windows ha sempre avuto questa numerazione a partire dal mitico Windows 1.0
Fonte: http://windowsvistablog.com/blogs/windowsvista/archive/2008/10/14/why-7.aspx