First things first. The most important part of the Style Guidelines is the subcategory description of a single beer style. This is much more important than any grouping into a category or style family. If there is ever any confusion about inferring some attribute by how a beer is categorized, always defer to the specific descriptions for the subcategory.
The groupings in the Style Guidelines are somewhat arbitrary, and often did not represent a unanimous decision of those on the Style Committee who worked on the document. There are two conflicting schools of thought represented in the guidelines. The first says that Style Guidelines should describe beer in the way you would think about it historically, or the way you would teach it in a study class. Similar to a Michael Jackson book, subcategories of beers should always belong to a logical style family from which they are derived. The other school of thought says that subcategory descriptions are the vital notion, and that style categories are simply logical groupings of similar beers for purposes of judging. This group believes that beers from many style families can be combined into judging categories so as to reduce the sensory differences a judge will encounter in judging a single flight from that category.
The effect of the first group can be seen in categories such as Stout, where a 1.040 Dry Stout can be judged alongside a 1.110 Imperial Stout. The effect of the second group can be seen in categories such as the Light Hybrid Beer, Amber Hybrid Beer, and Sour Ale categories, where seemingly unrelated beers are grouped according to sensory impact. Different members of the Style Committee believed different notions should prevail, so the groupings reflect a category-by-category compromise with majority voting making the final selection. There was much more discussion on the groupings of the styles than on the content of each individual style subcategory.
All competitions are free to group style subcategories in any way that makes sense to the organizer. Some competitions, such as the AHA National Homebrew Competition, do not collapse or alter style categories—they award medals by BJCP style category. Either approach can be used.