I love to work out.  I do it on vacation, road trips even when deployed to the Middle East working 12 hours a day 7 days week (3 years)  So with that being said..I never did 12 reps...stuck with 3 sets of 8-10.  So I'm just gonna say 12 seems like some magical point where my body's like wtf is going on?  I really feel the workouts now! i feel it at 11 im sucking wind lol.  its awesome so sad i wasnt challeging myself all this time.  can't wait to see what happens in 12 weeks