Your activity increases every 2 weeks or 14 days interval. This is the reason why in most cases activity is less than the post count.
The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Rough summary of what this means: If you post once per day on average, then you will eventually get the maximum number of activity points that you can get. Posting more than once per day on average is useless for increasing activity. The maximum number of activity points you can get is 1 per day, but your activity score only updates every two weeks. Therefore, it will take about 100 days to get 100 activity if you post once per day on average.
Activity is updated every hour.
Calculator created by @TheBeardedBaby:
https://docs.google.com/spreadsheets/d/1khSA28Ab6miwyknp3JvStcL0Yu05SIbGBDZDKcOLO6s/edit#gid=0after researching a hour i finally found this thread, it will help newbie like me