The other side's number looks invented and we can't say why
It needs taking apart line by line by somebody who builds these for a living.
What's happening?
Their claim has landed and the total feels wrong: too round, too big, grown at every submission. But feels wrong is not a response, and the people on your side who could take it apart line by line are running the job. So the number sits there, gathering authority, because an unanswered figure starts to look like an agreed one.
Built numbers leave seams. Rates with no source, quantities that reconcile to no measure, heads that quietly claim the same cost twice, arithmetic that does not cross-cast, escalation applied to sums that already contain it. An independent review finds the seams because it rebuilds the number rather than reading it, and the output is not an opinion but a schedule: what survives, what falls, what needs evidence they have not provided.
That schedule works in both directions. It is the basis of a reasoned counter-position, in negotiation or in a formal response, and it also tells you which parts of their number are good, which is what stops you spending the dispute fighting items they will win.
The solution
Their number rebuilt, so the answer has a basis.