From the current behaviour, it seems that 'deselectOtherItemsFirst' should really be called 'deselectAllItemsFirst', because it will also deselect 'this' item.
I presume this it's just done this way because it is easier, rather than for any specific reason... However, because of this, if you click on an already-selected TreeViewItem (at least in a TreeView that is single-select only) it will always deselect and reselect it - there doesn't seem to be a way to ignore this unnecessary change in selection without doing weird hacky workarounds.
It's not a major issue (I'm not even entirely convinced I care!), but since it is behaviour that somewhat contradicts the documentation, I thought I'd report it.