Why some package are kept back by default ?

[GloW]

Hello

In my fresh installed Xbian, when i got

Terminal

xbian@xbian ~ $ sudo apt-get upgrade
[sudo] password for xbian:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
libc-bin libc-dev-bin libc6 libc6-dev xbian-package-xbmc
0 upgraded, 0 newly installed, 0 to remove and 5 not upgraded.

Do you know why these package are kept back?