Firebase Realtime DB query returns both a list and a map when using startAfter

I have the following piece of code, that is simplified for the purpose of illustration:

 List<Name> _nameList = [];

_getNameList() {
    String dbPath = “names/“;

    var startAfter = _nameList.length - 1;

    final db = FirebaseDatabase.instance
        .ref()
        .child(dbPath)
        .orderByKey()
        .startAfter(startAfter.toString())
        .limitToFirst(4);

    final nameFuture = db.once();

    nameFuture.then((event) {
      List<Name> nameList = [];

      print(event.snapshot.value);

      if (event.snapshot.value != null) {
        for (var item in (event.snapshot.value as List<Object?>)) {
          if (item != null) {
            Name d = Name.fromRTDB(item);
            nameList.add(d);
          }
        }
      }

      if (nameList.isNotEmpty) {
        setState(() {
          _nameList.addAll(nameList);
        });
      }
    });
  }

It initially reads the first 4 Name objects from Firebase Realtime DB. And whenever it is called, I expect it to read the very next 4 Name objects and attach them to the _nameList as stated in the code.

IMPORTANT to note that keys to each Name object in the Realtime DB are as 0, 1, 2, 3, 4, and so on. If I am not totally wrong, this actually should make the life easier when using startAfter.

As you may recognize, I print out the values coming from DB, as print(event.snapshot.value);.

The first read works perfectly fine, where startAfter is set to "-1". Example output from print(event.snapshot.value);:

[{name: "John"}, {name: "Johanna"}, {name: "Maria"}, {name: "Steve"}]

Problem 1 When I run the function again, where the startAfter is now 3, I see the following output:

[null, null, null, null, {name: "Michael"}, {name: "Maradona"}, {name: "Pelle"}, {name: "Messi"}]

I expect the list to have the size of 4 but it includes 8 items. And the first 4 are null. Why do I have 8 items in the returned list, and why are the first 4 null?

Problem 2 When I run the function for the 3rd time, where the startAfter is now 7, I get the following output this time:

{10: {name: "Jordan"}, 8: {name: "James"}, 9: {name: "Rambo"}, 11: {name: "Brian"}}

While the first 2 calls returned lists, the 3rd call returned a map. Why does it all of a sudden return a map in the 3rd call? And I make the call with orderByKey() but the returned keys in the map are not in order; why?