The pricing formulas in Motor, the estimating engine I built for a water feature shop, did not come from the manual. I pulled 32 of them out of the JavaScript behind Aquascape's contractor calculator, the tool contractors actually use to bid jobs. The manual was sitting right there, official and free. Ignoring it was the best design decision in the whole system.
A vendor never ships a sloppy calculator
Why trust the calculator over the manual? Because of what happens when each one is wrong. If the manual sizes a pump wrong, a reader shrugs and moves on. If the calculator sizes a pump wrong, a contractor bids a job at that number, wins it, and loses money on the install. Then the phone rings. So calculators get fixed and manuals drift. Give it ten years and the two quietly disagree, and everyone in the trade knows which one to trust without anyone saying so.
A vendor will ship a sloppy PDF. They will never ship a sloppy calculator.
Documentation is what a domain says about itself. The artifacts money flows through are what it actually believes. Once you see that split, you cannot stop seeing it.
