Look again?
The pec has two heads. Sternal and clavicular, named after where they originate from. Sure they share a common tendon and both heads will contract in flat bench and incline. But in incline bench the emphasis is placed more on the clavicular head due to position on the arm in relation to the torso.
Just because two seperate heads of a muscle share a common tendon doesn't mean the same movement will impact both heads equally. Take you deltoid example. 3 heads originating from slightly different spots and all insert at the same point on the humerus. So why doesn't overhead press give you big rear delts? Because of the position of the arm in relation to the torso. Sure the rear delts will still contract in overhead press but not to the same degree that they do in reverse flyes or barbell rows and that's why they don't get big doing only overhead press.
I wasn't suggesting I can alter the shape of my clavicular pectoral head, I was saying I wanted to stimulate growth in it more than my sternal head to even them up. Which is why I changed to doing incline first, where I can lift a bigger weight for more rels and recruit more motor units in the clavicular head than I would doing it afterwards in a fatigued state. Which hopefully helps stimulate hypertrophy in the clavicular head.
Sorry if I was unclear with my statement.