If you get a 99% quality haircut or a 99% close-to-perfect meal, it’s better than good.
On the other hand, if the scrub nurse only does a 99% job of disinfecting the tools in the operating room, you’re still going to die of an infection.
Some projects respond very well to ordinary effort and 99% perfect. In fact, the last 1% isn’t worth the time, the effort or the focus.
Others fail if they’re not fully and completely to spec.
Knowing the difference between the two saves everyone a lot of stress and avoids needless cost and avoidable disasters. We come out ahead when we put our energy into the projects that reward our effort and choose to meet spec with the others.