When turning your hands during flys, you are peak contracting the whole muscle not only the inner part. The quadriceps are made of 4 different muscles, not 1 which is why you can stimulate the vastus medialis vs say the vastus lateralis. Biceps is 2 different bundles of muscle that have two different origin points and 1 common insertion point which is why you can somewhat isolate the different parts. THe diffence is, the pectoralis major is 1 muscle not 4 or 2. It originates on the clavicle and along the sterum and inserts on the humerus. The fibers do not run along the sternum (they originate from it) which is why you can somewhat do exercise for the upper and lower chest but can't separate the inner and outer. If the whole pec muscle grows, the area near the sternum will become bigger also but you can't just builfd the area along the sternum. It's science not myth.