# docker version
Client version: 0.6.4
Go version (client): go1.1.2
Git commit (client): 2f74b1c
Server version: 0.6.4
Git commit (server): 2f74b1c
Go version (server): go1.1.2
Last stable version: 0.6.5, please update docker
# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
lxc-docker
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
I found this post via Google search and since no one else has answered, I think that the reason is that it has something to do with the way docker is released as a new version of itself. Sort of a guess by looking at the availability of all the versions of docker on my system.