We only return areas as reachable if they can actually be reached by a human in real life, and this means the isochrone may have reachable areas that aren't connected to the main reachable area.
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For public transport, this is likely to be around train stations - the area surrounding the station will be reachable, but the connecting area between stations may not be as you can't get off the train between stops!
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For driving, the same thing can happen where the gap between junctions on a motorway may be unreachable because it is not possible to stop or get off the road at that point. By default we don't allow driving journeys to end on a road where you can't stop and continue by walking. This behaviour can be configured using the snapping parameter here.

Islands are more likely to occur towards the edge of the reachable area because it takes longer to get to these areas and therefore there is less time remaining (from the maximum travel time chosen) to continue the journey on foot. Hence why the gaps between islands are not reachable - there is not enough time left to walk to them.