There is not a whole lot of information on the Summerset Isles, as it has always been shrouded in mystery. However, the Pocket Guide to the Empire's chapter on the Summerset Isles, it mentions that, "The Queen of Firsthold, for example, is the Dunmer Morgiah, daughter of Barenziah and sister of the King of Morrowind, Helseth." Also, it states that the daughter of the King of Shimmerene has been assassinated. If we take a look at the map of the Summerset Isles,
http://www.imperial-library.info/sites/default/files/gallery_files/obcodex_summerset_isles.jpg , we can see that these are in fact cities. So, I am going to guess that these cities are the capitals of individual districts, and that these districts each have their own Kings. This could also be a reason for the seventy years of silence before the Great War, because of internal strife being caused by individual Kings.