Board Thread:News and Announcements/@comment-5774380-20150416171240/@comment-5774380-20150422123917

It seems that Oricon deletes the old statistics, that's why the sales of December are not stored anymore; but the stats of that link you provide were taken from there, and includes even a screenshot of the old stats, so I think this gives you the reason. I already edited the first message of this thread including that new information. Thank you for this excellent observation. :)