Flat and level are different problems
A floor can be perfectly flat and noticeably sloping. Floating floors care about flatness; doors and countertops care about level. They have different fixes.
Manufacturer tolerance is not advisory
Installing outside it puts the warranty at risk — the terms are the manufacturer’s, not ours — and the joints are the first place the defect shows. It is a specification, not a suggestion.
Why the cost is not known up front
What is under an existing floor is not visible until the floor is up. An honest estimate says that before it says a number.