So W3C is dividing it into HTML 5 and CSS 3 and its
.1 counterparts for the more experimental things which HTML5test.com look at.
Where did you get this from? The grandparent post is correct; after 2.1, CSS is divided into modules that advance individually. For instance, there is a Backgrounds and Borders module which has a level 3 spec and a level 4 spec (most of the level 4 stuff is not yet mature enough to implement). There isn't really any monolithic "CSS 3" thing, let alone "CSS 3.1".
Whatg tries to put it all together in html 5 and css 3. That is the confusion.
WHATWG's HTML spec is simply called the "HTML Living Standard", they don't use a version number at all. And the WHATWG doesn't specify anything in "CSS 3".