Why backports packages not deactivated

[T <mlist4suntong@yahoo.com>]

I've been following the "Debian Backports - Instructions"
at http://www.backports.org/instructions.html
to use backports so as to  _deactivate_ all packages from
backports.org by default. However, I noticed that at least one
package is automatically updated from sarge to backport. 

First of all, I've done all the settings required in the instruction:

	$ grep backports /etc/apt/sources.list
	# backports
	deb http://www.backports.org/debian/ sarge-backports main

	$ grep -1 backports /etc/apt/preferences
	Package: *
	Pin: release a=sarge-backports
	Pin-Priority: 200

This is the to-be-wrongly-updated package:

     $ apt-cache policy hplip-ppds
     hplip-ppds:
       Installed: 0.9.3-3
       Candidate: 0.9.7-3bpo1
       Version Table:
	  0.9.7-3bpo1 0
	     200 http://www.backports.org sarge-backports/main Packages
      *** 0.9.3-3 0
	     100 /var/lib/dpkg/status

It will be automatically updated from sarge to backport. 
Why? 
How can I prevent it?

thanks

FYI, Its "Reverse Depends" are:
  hplip-base
  hplip
  hpijs

And none of the above depends will be updated:

    $ apt-cache policy hplip-base
    hplip-base:
      Installed: 0.9.3-3
      Candidate: 0.9.3-3
      Version Table:
     *** 0.9.3-3 0
	    100 /var/lib/dpkg/status

    $ apt-cache policy hplip
    hplip:
      Installed: (none)
      Candidate: 0.9.2-2
      Version Table:
	 0.9.7-3bpo1 0
	    200 http://www.backports.org sarge-backports/main Packages
	 0.9.2-2 0
	    900 http://debian.yorku.ca sarge/main Packages

    $ apt-cache policy hpijs
    hpijs:
      Installed: 2.1.2+0.9.2-2
      Candidate: 2.1.2+0.9.2-2
      Version Table:
	 2.1.7+0.9.7-3bpo1 0
	    200 http://www.backports.org sarge-backports/main Packages
     *** 2.1.2+0.9.2-2 0
	    900 http://debian.yorku.ca sarge/main Packages
	    100 /var/lib/dpkg/status

thanks