For anyone else trying to figure out where the 7 came from – check out the Wikipedia article referenced in this Computerworld article
Some Windows watchers, however, questioned Nash's claim that Windows 7 would be the seventh iteration of the OS. The AeroXperience blog counted seven as of Windows Vista, and eight if the consumer-oriented Windows Millennium was included. However, only if kernel revisions are tallied, XP wasn't counted -- and Windows kernel was incremented to 7.0 for Windows 7 -- would that work, the blog argued.
According to the Windows timeline on Wikipedia, XP's kernel is tagged as 5.1, and Vista's as 6.0.