You are correct. You simply have so many posts, that as soon as it enters a new 2 week period, you immediately cap out with an additional 14 activity.
How is forum activity calculated?Activity is calculated with the following formula:
activity = min(time * 14, posts)
Activity is increased by staying logged in or by posting and is updated every 30 minutes. There is a maximum of 14 activity points every 2 weeks.