packages "kept back"

[Daniel Jones <ddjones@speakeasy.org>]

Running unstable.

Executing "apt-get upgrade," I get:

The following packages have been kept back
  ddd grace lib-xt-java libldap2 libxml-generator-perl
python-numeric python-numeric-tutorial task-sgml task-tex
wmaker 
0 packages upgraded, 0 newly installed, 0 to remove and 10
not upgraded.

Why are those packages "kept back?"  Is it because upgrading
them would break something else or what?  And is this normal
or a sign of something misconfigured on my system?

Thanks in advance,

Dan