Because next week there will be someone in the exact same position as you, who is one activity week away from a new rank. You're right in that it is not a 'fair' way to do it because someone who is just one activity period ahead of you is now a long way ahead, but your suggestion isn't the solution.
That's exactly it. The only thing would have been fairer would be a slightly more granular distribution of the initial merit points based on activity rather than just rank. As it has been implemented now I would think it would be very difficult to go back and change that. So let it be, some people will feel hard done by, but that's usually the way with change.