Maybe they’re great at building, poor at scheduling. If they’d +4 year’ed the initial timeline, they’d be “on time”?
We see this in the world of IT. A team will be set an X month timeline by a sponsor who is well in the knowledge that it will take Y months, but who strongly hold some belief (which I do not share) that giving less time, which will have to be extended through painful negotiation, will result in a shorter timeframe than if they’d given Y months expectation.
For major projects like Crossrail you also run into the politics of it. Politicians are short term incentivised. They have budget cycles. etc.