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.