Strange behavior of C# Dictionary

I have following simplified program

using System;
using System.Collections.Generic;
using System.Text;

class ItemClass {
    public int Id = 0;
    public int[] Childs = new int[] { };
    public int Count = 0;
}

class Class1 {

    Dictionary<int, ItemClass> _Items = new Dictionary<int, ItemClass> { };

    private void AddRecursive(int ItemID, int count, ref Dictionary<int, ItemClass> ItemList) {

        if (!_Items.ContainsKey(ItemID)) {
            return;
        }
        ItemClass _item = _Items[ItemID];
        if (!ItemList.ContainsKey(ItemID)) {
            ItemList.Add(ItemID, _item);
        }
        ItemList[ItemID].Count += count;
        if (_item.Childs.Length > 0) {
            foreach (int tmpItem in _item.Childs) {
                AddRecursive(tmpItem, ItemList[ItemID].Count, ref ItemList);
            }
        }
    }

    public void Add(int item, int[] childs) {
        if (!_Items.ContainsKey(item)) {
            _Items.Add(item, new ItemClass() { Id = item, Childs = childs, Count = 0 });
        }
    }

    public Dictionary<int, ItemClass> MakeList(int ItemID) {
        if (!_Items.ContainsKey(ItemID)) {
            return new Dictionary<int, ItemClass> { };
        }
        Dictionary<int, ItemClass> ItemList = new Dictionary<int, ItemClass> { };
        AddRecursive(ItemID, 1, ref ItemList);
        return ItemList;
    }

}


class Program {

    static void Main(string[] args) {
        Class1 class1 = new Class1();
        class1.Add(1111, new int[] { });
        class1.Add(2222, new int[] { 1111 });
        class1.Add(3333, new int[] { 1111, 2222 });

        Dictionary<int, ItemClass> items1 = class1.MakeList(3333);
        foreach (ItemClass item in items1.Values) {
            Console.WriteLine(item.Id + "  " + item.Count);
        }

        Console.WriteLine("");

        Dictionary<int, ItemClass> items2 = class1.MakeList(3333);
        foreach (ItemClass item in items2.Values) {
            Console.WriteLine(item.Id + "  " + item.Count);
        }

        Console.ReadKey();
    }
}

It has a simple task of counting items and showing a list of items and their count. When I call the MakeList function the first time, results are expected.

Expected:

3333  1
1111  2
2222  1

3333  1
1111  2
2222  1

Actual

3333  1
1111  2
2222  1

3333  2
1111  7
2222  3

As I am re-declaring the variable ItemList, I'm expecting to see the same result when I call the function for second time, however it is like the results from previous call is cached and re-used.

Why this is happening and why is this behavior? Is there any way to avoid it?