Most people have more posts than activity, but my activity always catches up with my posts ? 
This is the explanation : Post ≠ activity but you need to post to get activity every activity update.
This is the calculation how activity calculated :
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)
Activity is updated every hour.
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every hour.
, i didn't.