Because Activity is not the same as number of posts: not every post is added to activity amount. If a post is too short, if user to hurry to get activity too fast, if a section is more for fun than for real discussion, — there could be different reasons why some posts are not counted as activity. Just don't pay too much attention to that. Post when you want to communicate with other forum users, to get and share information and everything will come in it's time!
