Intent of Element::getElementsByTagNameNS spec

I'm expect this question has already been asked, but I could not find it in my search through the archives...

The spec for the getElementsByTagNameNS method of the Element interface strikes me as strange.  Particularly the part, "in a preorder traversal of the Document tree starting from this node".  I would have expected something more like the spec for getElementsByTagName, "in a preorder traversal of the Element tree".  What is the intent of including Nodes from outside the Element's subtree in the returned NodeList?

thanks,
Seth