I have researched this same question and found that your muscle response is very different when you cut chort at 6-8 reps.
From what I have read and studied, depending on how long it takes you to do one single rep, if you don't do more than 8 your muscles have not been stimulated long enough to release the agent or hormone it takes for gaining size. This is the principle on what olympic wreslters train on, so they can maintain there weight but still continue to gain strength.
Where as when you exeed your reps to 10 or 12 you begin to release more hormone and then your muscles are more prone to grow in size and strength.
Of Course this is only what I have learned in school, and read in various books, but that does not make it 100% true, you still need to experiment with your workouts and try different things, but this should start you off fairly well.