I believe it's the New Members forum which does not count posts, however there could be others. Posts are not counted in certain forums to prevent people from posting fluff to get their post count up, to post in the ad forums or reach their custom title.
I am wondering if it really improves quality because I don't know if it's known from most users.
Yes, it does. If we counted forums such as the new members forum you could have someone post ten times saying hello to different people and then go post an advert, that is no use to the community. At least by forcing them to gain their post count in other forums there is more chance of something useful coming out. And normally if what someone is posting is rubbish, it (and possibly their account access) will be removed.