apt pinning question

[John Bazik <jsb@cs.brown.edu>]

I thought I had a pretty good grasp of pinning, but I can't understand the
following behavior.  What I'm trying to do is, for a particular package,
always install the newest package version from either my local archive
OR lenny-backports.

My local archive is oldstable (same as lenny), and I have no target
release defined.

sources.list:
   deb http://mymirror/debian lenny main contrib non-free
   deb http://mymirror/debian-security lenny/updates main contrib non-free
   deb http://mymirror/debian-backports lenny-backports main contrib non
   deb http://mymirror/debian-browncs lenny local

preferences:
   Package: *
   Pin: release a=oldstable
   Pin-Priority: 750

   Package: *
   Pin: release a=lenny-backports
   Pin-Priority: 80

   Package: dovecot-imapd
   Pin: release a=lenny-backports
   Pin-Priority: 750

% apt-cache policy dovecot-imapd
dovecot-imapd:
  Installed: (none)
  Candidate: 1:1.2.15-1~bpo50+1
  Package pin: 1:1.2.15-1~bpo50+1
  Version table:
     1:1.2.15-1~bpo50+1+browncs 750
        750 http://mymirror lenny/local Packages
     1:1.2.15-1~bpo50+1 750
         80 http://mymirror lenny-backports/main Packages
     1:1.0.15-2.3+lenny1 750
        750 http://mymirror lenny/main Packages
        750 http://mymirror lenny/updates/main Packages

The priorities are equal, but the older version from lenny-backports
is the candidate.  I would expect the newer version to be installed.

This looks broken to me.

John