The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
So it does not jump automatically by 14. You also need to write at least 14 posts.
You can compensate for weeks you did not post all 14 by writing more posts in another week.
You can never compensate a 14 day periods that you did not post at least 1 post.
The post can be found here:
Activity & new membergroup limits