Why the last four points are the hardest
The first ninety-five points of quality are won by competent people doing careful work. The last few cannot be — and that is the whole difficulty. The defects that survive in a good operation are not the ones a careful person catches; they are the ones the system itself produces, regardless of how careful anyone is. They persist precisely because effort has already been maximised. This is why telling a good team to try harder does nothing for the residual defect rate: the people are not the cause. Closing the gap from 95% to 99% meant accepting that the remaining failures were a property of the operating model and going after them as systems problems, not performance problems.