Does ObservableCollection.RemoveAt() do two selections?

I am using MVVM pattern application, and I have a property that binds the SelectedIndex property of a comboBox.

This is my view model.

private int _mySelectedIndex;
        public int MySelectedIndex
        {
            get { return _mysSelectedIndex; }
            set
            {
                 _mySelectedIndex = value;
                 base.RaisePropertyChangedEvent("MySelectedIndex");
            }

When I do:

myObservableCollection.RemoveAt(1);

The selectedIndex property is set twice, the fisrt time set -1 and the second one 1, the index that has the element that I had selected when I try to delete the ítem.

I would like to know why the selected index is changed twice, I think that it would be just once.

thank so much.