Why can't the claim just be cost minus tender?

Because overspend on its own is not entitlement: to recover money the claim has to link each loss to a specific event the other side is legally responsible for, not to the job going badly in general. Taking final cost and subtracting the tender price, sometimes called a total cost or global claim, quietly bundles the other side's events in with the claimant's own tender errors, inefficiencies and supply chain problems, and because the single total cannot separate them, the standard defence answers all of it at once by pointing at the claimant's own failings. What survives is the slower, itemised construction: this event caused this effect and these costs, shown from the records, with the claimant's own contribution conceded openly rather than left to be discovered. The headline is usually smaller, but far harder to dismiss, and a recoverable smaller figure beats an eye-catching one that collapses under challenge.

Say a groundworks contractor tenders a road scheme at £800,000 and finishes having spent 1.1 million pounds, so the instinct is to claim the £300,000 difference as cost minus tender. But the £300,000 mixes three different things: the employer's late release of the site, which is the employer's responsibility, but also the contractor's own optimistic tender and a plant hire firm that let them down, which are not. Because the single figure cannot separate them, the employer simply answers the whole claim by pointing at the tender and the plant, and the round £300,000 is dismissed. Rebuilt event by event, the standing time from the late site release comes to a documented £110,000, with the tender shortfall openly set aside. That itemised £110,000 is smaller than the headline, but tied to one event the employer owns, so it is the figure that survives.