Apt keeps back?

[Erik van der Meulen <erik@avondel.xs4all.nl>]

I have upgraded my 2.0 box to 2.1 using apt. After some problems all
seems to be working well. One silly thing I cannot figure out. If I run
an:

  apt-get -f -s dist-upgrade

The output is:
  
  Reading Package Lists...
  Building Dependency Tree...
  The following packages have been kept back
    whiptail lynx util-linux ae
  0 packages upgraded, 0 newly installed, 0 to remove and 4 not
  upgraded.

I am not sure what the 'kept back' means and what the reason is. Any
suggestions?

--
  Erik van der Meulen <erik@avondel.xs4all.nl>