This first chart shows the range of beliefs in Christianity. I would guess that the author would not be attempting to show that each grouping is as large or historically important as the next one, just the spectrum of belief conveniently boiled down to nice categories. Then he zooms into a section to further discuss the messiness of it all.
I'm not quite sure everyone would like their assigned categories, but of course that is one of the issues of the whole emergent/emerging church what is the validity and way to self define ourselves. Traditionally people were assigned class, gender, place in society with some people busting out of the assignment. Today post-moderns see that everything up for grabs, if one does not like his or her gender, change it. If you don't like your social class, create a new one. (This usually involve a fashion statement with an accompanying genre of music, as if clothes really do make the man.) It is my opinion that a lot of this talk is somewhat more about associations and less about theology, but of course not exclusively and not for all individuals.
A few years back when I wrote my paper on this topic the Wikipedia article on the Emerging church was somewhat less developed. The defining mark of the Emerging church was that they refused to be defined. However, that has been sorted through and captured quite well quite well by the volunteers writing the Wikipedia article Emerging church. You may want to surf over and get reacquainted with the movement.