CNN article
I suppose they set the precedent back when they jumped from Windows 3 to Windows 95. and then back to low numbers after 2000 and XP, but it still seems strange... Oh! and there were some NT versions mixed in too, plus Millennium and Vista versions, so it's complicated...
Mind you, they also went from XBox to XBox 360 and then XBox One, so I suppose version numbers are not Microsoft's strongest point.