PPA package listed as upgrade when same version is installed

[Andrea Ratto <andrearatto_liste@xxxxxxxx>]

I made a backport of kvm from hardy to gutsy in my ppa. The package is
installed and works, but it keeps being listed as an upgrade by apt:

        andrea@andrea-laptop:~$ apt-cache policy kvm
        kvm:
          Installed: 1:59+dfsg-0ubuntu2.1~ppa1
          Candidate: 1:59+dfsg-0ubuntu2.1~ppa1
          Version table:
             1:59+dfsg-0ubuntu2.1~ppa1 0
                500 http://ppa.launchpad.net gutsy/main Packages
         *** 1:59+dfsg-0ubuntu2.1~ppa1 0
                100 /var/lib/dpkg/status
             1:28-4ubuntu2 0
                500 http://archive.ubuntu.com gutsy/universe Packages
        andrea@andrea-laptop:~$ sudo apt-get upgrade 
        Reading package lists... Done
        Building dependency tree       
        Reading state information... Done
        The following packages will be upgraded:
          kvm
        1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
        Need to get 0B/646kB of archives.
        After unpacking 0B of additional disk space will be used.
        Do you want to continue [Y/n]? n
        Abort.

Upgrading does not change anything.
Any idea why?
Thanks