I also think this is a bit frustrating. I will take the advice and email a suggestion for improvement. I understand the need to recalculate as the tree has changed but it will be a lot of waiting to add the 25 discoveries banked up, many of which are photos (no recalculation is required).
My opinion - this is a marketing ploy. I believe they are very well able to recheck the remaining instant discoveries in the list immediately once they have added one, but that wouldn't keep you subscribing now, would it? The idea is to keep your enthusiasm going so you come back every day and your tree isn't finished before your subscription renews.