I would accept any of the numerous ideas on the subject, without query. I merely reproduced an RSS entry. I myself have these notes,from about a year ago. I don't remember the source but it was probably somewhere deep in the Technet forums.:

The numbers are for NT based systems only. They split off from MS DOS operating systems at NT 3.1, which caused confusion, progressed through 3.5, 3.51, and 4 (4 was released parallel to windows 95). NT 5.0 was one of the most major releases of the NT OS, and was called windows 2000. Windows XP was almost a minor release designed to update some features and extend NT into the home consumer range. It merged the separate product lines back into one which created NT 5.1. XP is basically the NT kernel from 2000 with the user friendly OS features of the home line. Server 2003 and XP x64 are 5.2. It is designated as NT 5.1 and 5.2 (5.2 is Win XP 64, Windows Server 2003, and Windows Home Server.) Vista and Server 2008 share the NT 6.0 designation.

I've been browsing and found a few references to Windows 286/386. There are several different versions of the same story. I seemed to have missed that one through the years, but basically it seems that it was Windows 2 (or a version thereof) for the 286 and 386 machine architecture. Doesn't really seem to fit in anywhere but it certainly is listed on many sites -- But not on a Microsoft site, oddly!