Directories left on upgrade, files not removed on upgrade?

[Daniel Leidert <daniel.leidert.spam@gmx.net>]

Hi,

I need to make a design change to the docbook-xsl* packages. ATM the
situation is, that the HTML documentation from docbook-xsl-doc-html was
installed into /usr/share/doc/docbook-xsl/doc and there was a link
in /usr/share/doc/docbook-xsl-doc-html/. Now I need to change this and
everything gets installed into /usr/share/doc/docbook-xsl-doc-html/doc/
and a link /usr/share/doc/docbook-xsl/reference.html is created.
So /usr/share/doc/docbook-xsl/doc should disappear. But when I try the
update I receive this:

Preparing to replace docbook-xsl-doc-html 1.73.2.dfsg.1-5 (using docbook-xsl-doc-html_1.74.2-1_all.deb) ...
Unpacking replacement docbook-xsl-doc-html ...
dpkg: warning - unable to delete old directory `/usr/share/doc/docbook-xsl/doc/slides': Directory not empty
dpkg: warning - unable to delete old directory `/usr/share/doc/docbook-xsl/doc/lib': Directory not empty
dpkg: warning - unable to delete old directory `/usr/share/doc/docbook-xsl/doc/utility': Directory not empty
[..]

and everything in /usr/share/doc/docbook-xsl/doc/ is left. I (always)
thought, the upgrade process includes to remove the old package (its
directories and files)? Isn't it? So why does this fail? I tried to find
some more information about the upgrade process, but I just found a few
notes in dpkg(1) and of course section 6 of the policy.

Regards, Daniel