The estimate is material plus labour. Tonnage, sections, connections and the hours each one takes produce a price, and quoting from a rate per tonne alone means the complicated jobs subsidise the simple ones.
Approval is the milestone that stops the clock. Fabrication cannot begin until drawings are signed off, and a delay there moves the delivery date even though the workshop has done nothing wrong.
The workshop has a finite number of hours. Committing to a delivery without checking what is already in the shop is how three projects end up needing the same fortnight.
Material carries its paperwork. Mill certificates and traceability of what was used where is required documentation, and reconstructing it after delivery is close to impossible.