What Donald is saying is that if you aren't consistent about time and temperature, particularly of the developer, you won't get a consistent level of contrast in your film or prints.

Furthermore, with the right testing, you can control contrast in a creative way, because not all lighting conditions create the same level of contrast, and not all reflective objects in the world will be rendered with the same degree of contrast on film. In color work, you might also get color shifts.

For a basic understanding of how this works, read Ansel Adams' _The Negative_ and _The Print_. Adams is writing about B&W, but the principles can be applied to color as well, with the additional variable of color balance taken into account.