Here is an explanation of where the name "Windows 7" comes from.
From this article. Core of "Windows 7" taking shape: meet the "MinWin" kernel Windows 7
Why "Windows 7"? The number is based on Microsoft's internal operating system numbers: the first version of Windows NT, 3.1, was given the same number as the "Classic" Windows when it was released in 1992. Since then there has been NT 4, Windows 2000 (NT 5), Windows XP (NT 5.1), and Windows Vista (NT 6). You can check these numbers by typing "ver" at a command prompt on any of these operating systems.
But as I saw elsewhere, they're breaking their own naming convention because "7s" version number is NT 6.1. (Sorry I don't have the source on this.)