UnboundedPriorityMailbox doesn't keep message reception order

PriorityUnboundedMailboxTest.cs

using System;
using Akka.Actor;
using Akka.Configuration;
using Akka.Dispatch;
using Akka.Util;
using NUnit.Framework;
using UntypedActor = Akka.Actor.UntypedActor;

namespace TestAkkaPriority
{

    /* 
     App.config:
       <akka>
    <hocon>
<![CDATA[
my-unbounded-priority-mailbox {
    mailbox-type : "TestAkkaPriority.PriorityMailbox, TestAkkaPriority"
}
]]>
    </hocon>
  </akka>
     */
    [TestFixture]
    class PriorityUnboundedMailboxTest
    {
        /*
        Prints:
            Received message: 0-m0
            Received message: 0-m3
            Received message: 0-m2
            Received message: 0-m1
            Received message: 1-m0

        Expected:
            Received message: 0-m0
            Received message: 0-m1
            Received message: 0-m2
            Received message: 0-m3
            Received message: 1-m0
        */
    [Test]
        public void MailboxesDequeingOrder()
        {
            using (var system = ActorSystem.Create("Mysystem"))
            {
                var actor = system.ActorOf(Props.Create<SimpleActor>().WithMailbox("my-unbounded-priority-mailbox"));

                actor.Tell("0-m0");
                actor.Tell("1-m0");
                actor.Tell("0-m1");
                actor.Tell("0-m2");
                actor.Tell("0-m3");
            }
        }

    /*
    Prints:
        Dequeued 0-m0
        Dequeued 0-m3
        Dequeued 0-m2
        Dequeued 0-m1
        Dequeued 1-m0
    */
        [Test]
        public void PriorityQueueOrdering()
        {
            var queue = new ListPriorityQueue(10);
            queue.SetPriorityCalculator(o => int.Parse((o as string).Split('-')[0]));
            queue.Enqueue(GetEnvelope("0-m0"));
            queue.Enqueue(GetEnvelope("1-m0"));
            queue.Enqueue(GetEnvelope("0-m1"));
            queue.Enqueue(GetEnvelope("0-m2"));
            queue.Enqueue(GetEnvelope("0-m3"));

            while (queue.Count() > 0)
            {
                Console.WriteLine("Dequeued {0}", queue.Dequeue().Message);
            }
        }

        private Envelope GetEnvelope(string message)
        {
            return new Envelope(message, ActorRefs.NoSender);
        } 
    }

    class SimpleActor : UntypedActor
    {
        protected override void OnReceive(object message)
        {
            Console.WriteLine("Received message: {0}", message);
        }
    }

    public class PriorityMailbox : UnboundedPriorityMailbox
    {
        public PriorityMailbox(Settings settings, Config config) : base(settings, config)
        {
        }

        protected override int PriorityGenerator(object message)
        {
            return int.Parse((message as string).Split('-')[0]);
        }
    }
}