Lets take the cake analogy. Everyone can use the same ingredients, flour, eggs, sugar, water, etc. But not everyone's cake tastes the same. And certainly not everyone can bake a good cake. In this case Apple are using the different parts, CPU/Chipset/GPU/etc and designing the complete system around those parts. Seeing which part works with which and making sure that they have the "proper" drivers for the parts which they do use. They design their logic board which is very important. The length of the pathways and the layout of it does contribute to the performance of the system. So yeah in a way they do bake a good cake 