the way I see it, a user have zero points, then let's say posting a question today give the user 100, the overall score goes up by 100, the 'fresheness' (for lack of a better term), is also 100. The next day the doesn't take any actions. Overall scores remains 100, but the 'freshness' scores goes down by a factor (let's assume a 10% drop per day), so it is 90 now.
The third day user post/answer another question, overall scores goes up +100 and is now equal to 200, the freshness also gets a +100, and it is now equal to 180 (we had another -10 drop as another day has passed).
Yet if another user makes 2 post the third day, both will have the same overall score, but user2 will have a slightly higher freshness score.
We might not need to expose the freshness score (which calculations might be difficult to grasp for regular users), but as an indicator (icon, color bar,under the user avatar, etc).