What do you mean by moving target? DV visa numbers? Or NACARA figures?
I think they might get NACARA data with some delay. But KCC/NVC is the authority that issues visa numbers, so consulates can issue visas only after they get visa number from NVC (I think for DV it is KCC but I am not 100% sure). So, if you are talking about NACARA numbers as moving target, they are moving slowly. And it is even much more slow after you take 7%.
If you are talking about DV visa numbers, they are supposed to issue them at KCC/NVC first. Then consulate could deny a visa, and those numbers go back to KCC for recycling to be assigned to someone else. So, when KCC says all Iranian visas are taken, that means no recycling is possible (so, visas were actually issued), and the number of NACARA cannot decrease. Also, they need to have a procedure to make sure if NACARA increases, the 7% limit that has already been hit does not go down because of that.
Somehow, by Sept 20th they could be sure no more NACARA would be issued - too little time left, and USCIS might tell them the figure is final.
If that all happens differently, not the way I just described, why without NACARA they had 3859 max? If it is difficult for them to track precise number of visas, why they did not have major differences (more than 9) versus 3850 when NACARA was not present? Am I understanding you wrongly? Is not it true that all you are saying should be equally applicable for the years prior to 2000 (when NACARA was not part of equation, and they had just one moving target instead of two)?