ChoiceBox docs state "it is possible for the developer to specify the selected item to be something other than what is available in the predefined list".

Example code:

public class ChoiceTest extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        ObservableList<String> list = FXCollections.observableArrayList("A", "B", "C");
        ChoiceBox<String> choiceBox = new ChoiceBox<>();
        choiceBox.getSelectionModel().select("D"); // or choiceBox.setValue
        choiceBox.setItems(list);

        stage.setScene(new Scene(choiceBox));
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

The result is that the displayed item is blank. A quick look shows that ChoiceBoxSkin is set to display "" when selectedIndex = -1, which is the value for a selected item which is not in the list of items.