The way I see it. The main lift is constant because you need something to evaluate your progression objectively as well as to assess your weaknesses.
Now the Power Look program is a generic one. In reality the way you should work is:
- Do the main lift as planned
- Use the performance on the main lift to evaluate what are your weaknesses (part of the range of motion, a specific muscle group, etc)
- From that evaluation you select your assistance exercises for the day.
So the assistance work can indeed change at every session if needed. It has to be something that fix what you feel is holding you back in the main lift.
So the main lift helps you select the exercises that will stimulate strength and size growth the most by addressing something that is lagging.
Then you use the progression on the main lift to evaluate if your exercise selection is adequate... if you are not progressing then you might not be selecting the proper assistance work.