The End map
Until I get some hard data, I'll estimate there's something like an 85% probability of a city being located where a "Likely End City" is marked, but perhaps only a 10% probably where the Possible End Cities are marked.
Amidst doesn't fully generate the Minecraft terrain, instead it shows a map outlining where and how Minecraft will generate terrain.
The End map shows all the locations where an End city might be located, but the landscape must reach a certain height for an End city to form, so without fully generating the terrain Amidst can't know for certain if the landscape will support a city.
However, Amidst still has a pretty good idea of whether the landscape will support a city, thus "Likely End Cities" vs. "Possible End Cities".
Unfortunately the miniature islands aren't determined by the world's seed - they depend on which parts of the End you have visited, and in which order. For example, approaching the same location from different directions leads to this:
So the End map shows a miniature island pattern to indicate the areas where they can be found, rather than showing actual positions.
##Why are all the islands circular?
The End map is showing the "cores" of the large End islands, in reality the island coastlines won't be perfectly round and may even join together to bridge multiple cores together into one giant island.
The End map gives a good idea of the location and shape of the large land masses, but not the coastlines.