1 post = 1 activity point and if someone who has 120 activity points and 200 posts and suddenly that person decides to delete 120 posts then he/she will have only 100 activity points
This is not accurate.
Your activity is minimum of 14*number of two-week periods in which you have been active and number of your posts. So, if you delete some of your posts and your post count decreases to 100, your actvity wouldn't be necessarily 100. Your activity will be a number between 0 and 100.