Last modified: 2013-03-18 13:26:09 UTC
E.g. you get some of the "military" categories with http://en.wikipedia.org/w/api.php?action=query&list=allcategories&acprefix=military and some with http://en.wikipedia.org/w/api.php?action=query&list=allcategories&acprefix=Category:military
The definition of "category" for list=allcategory includes anything that was ever a category on a page, even if it doesn't have any members anymore. The ones showing up with "acprefix=Category:military" were categories like "Category:Category:Military and war museums in Oregon" with a doubled prefix.