apt-get package listing question

[Brian Nelson <nelson@bignachos.com>]

I'm new to Debian, and I have a question about apt-get and why some
files are missing from its listing.

Recently, I needed to use unzip, but it wasn't installed on my system. 
So I tried 'apt-get install unzip', but it said the package was listed
in the database but was empty/obsolete/or whatever.  Confused, I tried
'apt-get install zip' and that worked fine.

So I went to the package search engine on debian.org and searched for
unzip.  It found copies in every distro (stable, testing, unstable).  So
I downloaded the deb and installed it.

But, why wouldn't it show up with apt-get?  I had done 'apt-get update',
and had a couple different listings pointing to testing in sources.list.

Thanks,
Brian