It's important to realize that development and design are two completely different tasks. In a perfect world, the development would always match the design perfectly. After all, you've put a lot of effort into the task of designing the application, so it would be nice if the application looked precisely like your design after you complete the development. In the real world, however, things change during development. The perfect design is no longer so perfect after all. The application runs too slowly, doesn't meet specific security needs, or seems to fail a little too often. These three issues are related to the performance triangle:
During the design process, you worked through the theory of the performance triangle. The development process also requires that you rely on the performance triangle. However, now you're putting the theory into practice and you may find that you must make some changes. Every change you make should reflect good coding practice and you should quantify each change to ensure the change is necessary.
Notice that the chapter doesn't mention anything about new requirements, additional features, extra reports, or anything of that nature. Starting the development process doesn't give anyone carte blanche to make changes of this sort. In this case, you should stick by the design and make changes as updates later to keep the project on track. You don't want to be completely rigid, but ...