Its confusing but Interesting too
My case was reviewed my NVC on 15th and sent to NWD on 19th of march, but the letter I got from NWD consulate shows the priority date as March 12th.
I think when NVC sends the P3 to the condidate, same time it also sends to the counslate. So counslate gives the priority date based upon the time it got the P3 from NVC and keeps on waiting for the P3 from condidate too. As in some of our case it took time to reach the P3 to the consulate(as per new process it has to go through NVC), so they issued new case # as they thought key can\'t keep the file forever.