Firebase returning keys of child node in different orders on different devices/Android versions

I am getting a snapspot of the data from my Firebase database to retrieve a users list, but the order of the keys being returned is different depending on the Android version/ device being used.

For demonstrative purposes I have shortened the method, but it is essentially as follows:

public void getUsers(){

Firebase ref = new Firebase("http://ift.tt/1eQ0zaI");

    final Firebase userRef = ref.child("users");

    userRef.addListenerForSingleValueEvent(new ValueEventListener() {

        @Override
        public void onDataChange(DataSnapshot snapshot) {


           snapshot.toString();

                                                           }                                                           
                                                                  });
                           }

It is the data I get from calling toString() on the snapshot object (snapshot.toString()) that changes order.

I have tried it on 4 devices. The 2 running Lollipop (Nexus 7 5.1.1 & Galaxy s4 5.01) return the data in the same order. And the 2 two other devices (HTC Sensation 4.0.3 and Motorola G2 4.4.4) return the data in the same order (but a different order to devices with Lollipop).

There is no difference in the code used, and the data in the database was completely unchanged at the times when I retrieved the snapshots.

Here is the data order on the 4.4.4 and 4.0.3 devices:

DataSnapshot { key = users, value = { 114585619420240714499={userIDOfCUser=114585619420240714499, NameOfCUser=testName, EmailOfCUser=testerfireapp@gmail.com, friends={103902248954972338254={userIDOfFriend=103902248954972338254, NameOfFriend=testName2 }}}

Here is the data order on the 5.1.1 and 5.01 devices:

DataSnapshot { key = users, value = {114585619420240714499={NameOfCUser=testName, userIDOfCUser=114585619420240714499, friends={103902248954972338254={NameOfFriend=testName2 , userIDOfFriend=103902248954972338254}}, EmailOfCUser= testerfireapp@gmail.com}}}

Why is the data being delivered in different orders depending on the android version/device being used? is there another difference I am unaware of ?

Thanks in advance.