This is still very much speculation, but is it possible the Jetstar 787 deliveries are being deferred?
The thing is, the 787-8 can't operate on all the long routes the A330 operates because some genius ordered the 787-8 with the wrong engine pylons. Im not sure how many 787's Jetstar has currently, but just on the top of my head, the 787 wont be able to fly to Honolulu, or Melbourne to Narita, Melbourne to Singapore. So thats 4 less 787's jetstar needs.