Quoting numerarius5988am, reply 54If an American moves to Australia, then they'd still be American. Also, sometimes people refer to their roots or ancestry.
what about his grandchildren? his grandchildren's grandchildren? are they american? i am not polish, despite the fact that my grandmother is from poland
Hence the reference to ancestry. The Advent prefer desert worlds because of the history of their ancestors. I am American, but I could just as easily refer to the component ethnicities of my ancestors: French, Irish, English, Scottish, Italian, and so forth. My last name is French, Fournier, so I could most easily give myself the appellation of "French" in a sense if I wanted to. However, the Advent in the game would probably want to associate more with their ancestors which were exiled from their desert planet, rather than associating with the rest of their ancestors who exiled them.