as i know. from the first day you create your account your activity is only up to 14 activity, although you post more than 14 post your activity wont added until the next update. update is only happen every two weeks.

so on your question that How exactly do I have 5 activity in my first day? just make a post but not in burst, make your post more constructive and informative. then you will see that your activity is going up until it reach 14 activity.