[CSS21] Clipping abspos elements

I got a question about this bit of text in 11.1 <http://www.w3.org/TR/CSS21/visufx.html>:

   # In certain cases, a box may overflow, meaning its content lies
   # partly or entirely outside of the box, e.g.:
   # ...
   # A descendent box is positioned absolutely, partly outside the box.
   # Such boxes are not always clipped by the overflow property on
   # their ancestors.

Can't figure out what that "Such boxes are not always clipped" sentence
is talking about. An implementor was wondering if it meant abspos
descendents are /not/ supposed to be clipped by "overflow: hidden".
I'm quite sure that's not the case, so, perhaps we could either clarify
what is meant or remove the sentence.

~fantasai