The prog spectrum is an attempt to categorize the whole body of prog music in a meaningful way. Many of the categories are really broad and inclusive, some others are rather specific. The latter are included for historic reasons. Genres like RPI, Krautrock and Zeuhl are so well established that it simply would not make any sense to ignore them. The genres are listed in a particular order which attempts to make sense both in a chronological and stylistic way, grouping similar categories together. This only works to some extent of course, there is no perfect solution.
NOTE: Releases can and do appear in many different sections of the spectrum. Many prog releases cover several styles and genres, which is one of the reasons why we like prog so much. When you see a release in one of the categories, you should think of that as "this release is ALSO ..." rather than "this release is ONLY ...".