I see two types of "Islands", those with connections and there is NO room for them in the main tree, which has a linked box to the island, and those without ANY connection, which will normally appear in the lower left corner of the overall tree. These none connected island can be 1 or many people.
Yes, this will ONLY happen IF all individuals are included.
See that attached, it a database that I have been working on, based on the "Residents" of a local cemetery of a Coal mining town. It is a attempt to show how the town is interrelated. Only about a third of the persons in the database are buried at the Newcastle Cemetery. I have done some searching and have found the links from other small mining towns and early census.