Just simply put, if you post at least 1 post every 14 days you'll get 14 activity, if you post 1 time in 2 periods (28 days) you'll still get 14 activity and not 28 so you must post 1 post every 14 days to get your activity. It doesn't matter when you post in that period, you can post at the end of the period and next day at the beginning for the new period and you'll still get 28 activity for 2 periods.
You get 14 "time" which in turn is used when activity is calculated, but the number of posts is still a cap on your activity. Thus 1 post in 14 days will result in 1 activity unless you previously had # posts > activity.