Because 2.4 is still marked as alpha. Versions on the torproject download page, their repo and Ubuntu repo all match.

    % wajig policy tor
    tor:
      Installed: 0.2.3.25-1
      Candidate: 0.2.3.25-1
      Version table:
     *** 0.2.3.25-1 0
            500 http://ua.archive.ubuntu.com/ubuntu/ raring/universe amd64 Packages
            100 /var/lib/dpkg/status
         0.2.3.25-1~quantal+1 0
            500 http://deb.torproject.org/torproject.org/ raring/main amd64 Packages

Depends on the distro and the version.