For each day that has passed, visiting that day is worth 3 points and the first four posts are each worth 2 points, for a total of 11 possible points for each day.
After these points have been calculatated for each day to be included (14 days, right now), they are multiplied by a week weight. The most recent week is multiplied by 36. The week before is multiplied by 27. Two weeks ago is multiplied by 21. Three weeks ago is multiplied by 16. Four weeks ago is multiplied by 12. Five weeks ago is multiplied by 9.
After the week weights are applied, the final scores are added up and compared against the maximum score possible. The percentage is your activity.