They fill pre-orders in the order received. So they must not have received enough stock to fill all pre-orders. You're in the queue waiting for 2nd batch delivery.
Pre-ordering is never a guarantee that you'll get your order on release date at HLJ these days.
I always try to pre-order as soon as something is posted, and this time fortunately I was early enough in the queue to get a batch 1 fulfillment of my order,
It's probably time to start thinking about requesting shipment for me - in addition to the YF-21 w/ booster, I have a plamax VF-1J battroid, VF-1J fighter, 2 VF-1A fighters, a Queadluun Rau Max, and a YF-19 battroid waiting to ship.