I think we've touched on that a bit, but I am happy to elaborate, and maybe Pete will want to jump in here too.
It's because the specifications changed. The articulation in the early years of the project was that the joint project office required a certain kind of hangar, and it's much more than just a hangar where we park the aircraft at night. That change to require much greater security includes those steel plates that Pete was talking about earlier.
That then meant a much longer and more expensive supply chain to ensure that the steel that makes it into the hangar has been protected from birth, so to speak, all the way along that supply chain. Strictly speaking, security costs alone would have gone up because of the need to make sure the entire supply chain was secure, so we didn't end up with steel in the building that had other things in it, for instance.