New Post: SelectedItem --- is there some bugs?

In the documentation
http://wpftoolkit.codeplex.com/wikipage?title=CheckComboBox
it says:
SelectedItem
Gets/Sets the last checked or unchecked item. (Inherited from Selector.)

However, with the below code:
private void CheckComboBox_CostCenter_ItemSelectionChanged(object sender, Xceed.Wpf.Toolkit.Primitives.ItemSelectionChangedEventArgs e)

    {
        string selectedItem = CheckComboBox_CostCenter.SelectedItem.ToString();
        int selectedIndex = CheckComboBox_CostCenter.Items.IndexOf(selectedItem);              
     }

I am always getting the first checked item, even when the second item is checked (when the first item still remained as checked). Should it go to the second item, since it says it gets te last checked item?