the fiddle example shows that adding a selected item to a listbox, works for appendChild and insertBefore but not when using listitem.setParent()
http://zkfiddle.org/sample/ij4ufp/1-listbox-add-selected-listitem (need to clear the selection before each insert, to see that setParent has no effect)
enabling multiple selections makes this more apparent. no need to clear the selection here, just append/insert/setParent items and see that setParent "initally" has no effect and only updates when another item is added.
use appendChild() and insertBefore()