A borrowing figure produced by a calculator is arithmetic. The decision it resembles is made by somebody else, under obligations set in law, using information no form has collected.
Australian credit law is specific about what that involves. Before offering credit, a licensee must make reasonable inquiries about a consumer’s financial situation and about their requirements and objectives, take reasonable steps to verify that financial situation, and then assess whether the contract would be unsuitable for that consumer. Where the licensee is the credit provider, that final assessment is its own to make. None of the three is arithmetic. Inquiry, verification and assessment are activities involving a person’s actual circumstances, and a page that receives four numbers has performed none of them.
The same gap runs through the other outputs, in different forms. A repayment schedule describes what an unchanged rate would produce, which is an assumption rather than a statement about the future. A duty figure applies published rules to stated inputs and is not an assessment by a revenue office. In each case the calculation is sound and the thing it resembles is a decision that has not been made.
The inputs are therefore the durable part of any calculation and the output is the disposable part. Whoever makes the real decision recomputes the number from information of their own; what carries across is the set of assumptions, which is also what makes two honest answers to the same question differ.