It also directly implies that something (some parts, software or whatever) is not ready when they leave the factory. In other words Derby is not making them to handover condition.
Basic commercial reality is train builder would try and supply them, get them handed over and get paid. Manufacturers normally only stockpile if delivery is running early, and before customer is ready.
We know the first ones are about 29 months late and final ones 14 months late, so the storage not in customers sidings, means not ready to be handed over. If they were then somewhere like new Feltham depot would be awash with them.