Cannot install from sarge-backports

[backports@itmaze.com.au]

Hi all, been hunting to find out how to install a package from backports
with testing and unstable sources also in my sources.list.

The following command:

   sudo apt-get -t sarge-backports install openoffice.org


Gives me this error:

   The following packages have unmet dependencies:
     openoffice.org: Depends: openoffice.org-base (> 2.0.1) but it is
   not going to be installed
   E: Broken packages


My sources.list says:

   deb http://www.backports.org/debian/ sarge-backports main
   deb http://mirror.aarnet.edu.au/debian/ stable main non-free contrib
   deb http://security.debian.org/ stable/updates main contrib non-free
   deb http://ftp.debian.org/debian/ stable main non-free contrib
   deb http://mirror.aarnet.edu.au/debian testing main contrib non-free
   deb http://mirror.aarnet.edu.au/debian-security testing/updates main
   contrib non-free


My apt-preferences says:

   Package: *
   Pin: release a=testing
   Pin-Priority: -1

   Package: *
   Pin: release a=unstable
   Pin-Priority: -1

   Package: *
   Pin: release a=sarge-backports
   Pin-Priority: -1


The output for:

   sudo apt-get -o Debug::pkgPolicy=true -s install -t sarge-backports
   openoffice.org | grep backports

Is this:

   Prio of

/var/lib/apt/lists/www.backports.org_debian_dists_sarge-backports_main_binary-i386_Packages
   990
   Prio of

/var/lib/apt/lists/www.backports.org_debian_dists_sarge-backports_main_binary-i386_Packages
   990

(The rest of the output are priority 500 for stable, 100 for installed
and -1 for the rest.)

My understanding is that I should be able to specify the source with the
"-t {sourcename}" and that any package that gets installed will also
install any dependencies from that same source.

What am I missing?