apt expectations

[Jim McCloskey <mcclosk@ling.ucsc.edu>]

I have a system that is mostly debian testing. But I have apt set up
so that apt-get update fetches information about both the
testing(woody) and unstable(sid) archives. I have /etc/apt/preferences
set up thus:

Package: *
Pin: release a=testing
Pin-Priority: 900

Package: *
Pin: release o=Debian
Pin-Priority: -10

I tried to upgrade to the new mozilla by doing:

   apt-get install mozilla-browser/unstable

But apt complained about an un-met dependency (a too-old version of
libnspr4). I fixed this by way of:

   apt-get install libnspr4/unstable

after which `apt-get install mozilla-browser/unstable' successfuly
installed version 0.9.7 of mozilla from sid.

But it seems to me that apt-get did not behave according to
expectation here. I thought that:

   apt-get install mozilla-browser/unstable

would automatically bring down the packages from unstable that the
version of mozilla-browser in unstable depended on.
   
Was I wrong in this expectation, or is there something wrong with the
way that dependencies are calculated for mozilla-browser in the
unstable distribution?

Jim