It may be that once you hit the 14 limit, then you have to wait for 14 days if you are a prolific poster. I hadn't considered that.
It is this formula.
activity = min(time * 14, posts)
That means the minimum of the two values time (the number of two week periods you posted in) multiplied by 14 or the total number of posts.
So you signed up on the last day of an activity period giving you a maximum activity cap of 14 and then when you posted the next day which was the next activity period that cap become 28.
If the number of activity periods you posted in multiplied by 14 is more than your total posts then you have 'potential activity' and can increase activity just by posting.
If the number of activity periods you posted in multiplied by 14 is less than your total posts then you have to wait until the next activity period to increase your activity.