I don’t know. Two of my nieces were born in Britain and I was told that had they been born at the base hospital would have been American citizens, period. But since they were born in an English hospital they became dual citizens.
Dual Citizens? Yes, but DUAL means two.
They are US Citizens whether they were born on the base, in a British Hospital, or on the side of the road.
Maybe because England recognizes a dual citizenship, but the United States does not. In other words, your two nieces were already U.S. citizens at birth, but because they were born on British soil at the British hospital, and not on the U.S. base (U.S. soil), they were given British Citizenship, as well.