Activity is calculated in the following fashion and increases on different days for different accounts as a result of each person registering on different days.
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 30 minutes.
Your probably out by a couple of hours, but it will definitely increase. Trust me, it's highly unlikely you've discovered a bug exclusive to your account - much more likely you've got your times mixed up. It is also completely irrelevant how many times you post as long as your post count is > the number of two week periods you've made a post.