WordPress has four different datacenters and this could be an issue related to mirroring from one center to another simply not being caught up yet. In fact this tends to happen on the first day of every new month.
All of my blogs are still showing November only. I just ignored this last month and it was resolved in time. However, if you wish you can always send in a feedback to staff. http://wordpress.com/forums/topic.php?id=4997&replies=15#post-32161