vertical spacing confusion

[Mark Polesky]

It seems that all the \header variables get grouped into one
"title" block, except "piece" and "opus", which seem to be
grouped together into one "top-level markup".  You can see
what I'm talking about by adding:
  \paper { annotate-spacing = ##t }
to the lilypond example found in:

NR 3.2.1 "Creating titles"
(Documentation/83/lily-f0fcb803.ly)
http://git.savannah.gnu.org/gitweb/?p=lilypond.git;a=blob;f=Documentation/notation/input.itely#l599

...and then compiling it.  Everything from the dedication to
the arranger is in one "bottom-of-extent" block and the
piece/opus pair are together in another.  Is this
intentional?  If so, what is the reason for this?

Also, is there no better label than "bottom-of-extent"?
I think it's poorly named, and the label appears on more
than one type of annotation: one looks to be a
markup-extent, and I don't know what the other is; it goes
from the middle of the lowest staff in a system to the
lowest skyline or something?

Thanks.
- Mark