This is correct. The general rule is that the chart begins with the blood relatives of the selected person.
Then, for reports with an ancestor near "top-of-the-line is selected, spouses have extended family entered, an island is created for that extended family circle. And, if there are spouse in that island with extended relatives entered, there are more islands.
Every once in awhile I've noticed where there is only like one parent entered, the chart knows how to "cheat" and not create an island only two people - but, generally, the scheme is as described above.
And on and on it goes... to (almost) infinity.
Following are three examples of how the EFC works.