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.

I had the same dilemma few months ago when I started using ninjastic space and I also noticed the difference in numbers compared to merit dashboard. Thing with ninjastic.space (that I didn't know but DdmrDdmr explained it to me) is that is also counting all those posts that were for some reason deleted from the board, and that's why number is usually inflated compared to numbers found in merit dashboard. For example, if you had some spammer who wrote 20 posts in Indonesian board during March, and his account got nuked and all posts deleted, his posts will still be counted even though they are not there anymore.

With that being said, numbers on merit dashboard can also be wrong in case that mod deletes/moves threads around, but I decided to stick to merit dashboard numbers in the end as it doesn't seem right to count spam posts that have been deleted for days or even weeks.

So, in the end neither Merit Dashboard and ninjastic.space are 100% correct, and some weird results will happen, but I decided to stick with the former when making those charts as I prefer occasional odd results due high amount of deleted posts over inflated numbers. To get clearer picture of board activity, best is to use both of them.