Currently the CheckedItems collection contains items with CheckState "Indeterminate". The change can be controlled somehow since the modifying the behavior now will be a breaking change.

Please avoid using this collection, it is has never been implemented completely.
In DataBound scenarios, add CheckBox in ItemTemplate like it is shown in this help article.
http://docs.telerik.com/devtools/wpf/controls/radtreeview/how-to/howto-tri-state-mvvm.html