DOCBOOK: curious about duplicated elements

as i peruse all of the elements, i was curious about the fact that
a <book> can directly contain a <title> element, but a <book> can also
contain a <bookinfo> element, which itself can contain a <title>.

  this kind of duplication seems to be fairly rare -- a <book> can't
directly contain, for example, an <authorgroup>; that has to be
within the <bookinfo> element.  any reason why title, subtitle
and titleabbrev have this extra flexibility?

rday