Dammit! It all were in the mind and I did not crosscheck before making the post. Was I right about Investigation board?
I just made a search to find the answer and couldn't find anything. But it's seems that posts made in "Investigations" board are counted.
For instance,
notblox1 has made
this post in "Investigations" board. If you check his post count, you will see that his post count is equal to number of posts displayed in his post history. If posts made in "Investigations" board weren't counted, his post post count would be lower.