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.
Basically, it doesn't matter that you have more posts than activity points because you're still entitled to a maximum of 1 activity per post per day, whether you post more or have more posts is irrelevant.
Activity, as the name implies, requires that you be active. If you have been inactive it's only natural that you don't get any activity points.