How does apt-get construt package uri?

[Try KDE <trykde@rogers.com>]

Thanks to Eduardo Pereira Habkost's answer to the previous question - it works beautifully.

Here is a harder question (in my opinion, anyway): given a http/ftp line in sources.list, what's apt's algorithm for retrieve the list of packages. For example, 
   deb http://http.us.debian.org/debian/ testing main non-free contrib

will produce such an output from apt-get:
  Get:1 http://http.us.debian.org testing/main Packages [2036kB]

So exactly what uri was constructed out of it? My browser says http://http.us.debian.org/testing/main doesn't exist. By the way, apt-get's manpage is very unclear in this aspects.

Thanks,
-tk