Activity is something based on posts so it's kind of related but at the same time it doesn't mean that if you post more you gain more activity.However,if you don't post for let's say 6 weeks you will not gain any activity.If you have posted in just 1 of those 6 weeks,no matter how many posts you made in that 1 week you will still gain a limited activity based on a certain period which will not be for the whole 6 weeks.I hope I explained it at least somewhat understandable
But like hilariousandco has stated, If I made 154 posts now I'd get my activity straight to 154, regardless of the 14 points limit. Whilst, if I made a new account today and managed to get to 154 posts on there, my activity will stop at 14. Basically, since I haven't been active for months, and since I haven't got 14 points during those weeks, now I still have the possibility to earn those points.