[agent5150]

Why am I getting kept back message on upgrade attempt?

# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
xbian-package-config-xbmc xbian-package-upstart-xbmc-bridge
xbian-package-xbmc xbian-package-xbmc-scripts xbian-update
0 upgraded, 0 newly installed, 0 to remove and 5 not upgraded.


My sources list:
deb http://mirrordirector.raspbian.org/raspbian/ wheezy main non-free
deb http://apt.xbian.org stable main
deb http://apt.xbian.org staging main

Is this normal or am I missing something here?