In a world where large infrastructure projects regularly exceed their budget by 100-1000%, 7% is huge.

It is important to show people that that is possible in government projects.

If you find a more concise way of saying „with unusually small overrun of it‘s budget“ tell me.

Also there were sections added after the initial estimate.

€5.5 billion in 2005 is €8 billion in 2025, so it can be either over or under budget depending on how you amortize the costs over the construction period.