It's sometimes hard to guess their motivations, but we do know that it isn't simply a matter of price. While nothing over $4 is included, many things under $4 aren't. Also, the only things that are assuredly consistent from location to location are the things listed in the brochure.
There have been reports that some of the CS locations (not TS restuarants) do have some means of allowing you to come back and get your dessert after you've finished eating your meal. However, that's not part of the Dining Plan itself, but just something that some of the eateries are doing as a special thing for their guests.