I have a XamComboEditor bound to an ObservableCollection. As each item in the list is selected, inside the SelectionChanged event handler, the SelectionChangedEventArgs.RemovedItems contains all the existing selected items, however when deselecting an item in the list the RemovedItems only contain the item I actually removed as expected. Why does this occur? I had to wrap my remove code in a check to only run when the AddedItems count == 0.