The higher the rank, the more merits you need. Also you need minimum number of activities and merits to attain each rank... So, merit is not just the thing that you need for ranking up.
You can have maximum of 14 activities each week (please correct me if I'm wrong), meaning if you have more than 15 posts on that time frame, only 14 will be added to your activity count. This is also the reason that most users here has more post count than activity count.