As I said before, topics moved/deleted don't have to be new ones to affect post count difference, inactive old topic from few years ago that got moved will affect numbers. My assumption that such a big difference is probably caused by mods work is based upon previous experience in similar situations.
If we take into account the cases with the deletion of old posts, then I have a question, why take into account old posts and topics when it comes to the number of posts for a specific fresh month. Your statistics do not reflect monthly activity, they simply calculate the average between what was and what is. And shows the result. If the moderators try, then I will not be surprised that we can see in your statistics something like: 100 posts per month were published in the Russian locale.