What makes a MacBook much thicker or thinner? Apple's official website says the height of a MacBook could be from "0.35 to 1.31 cm". This is a wide range, more than double thickness.

I'm buying a new customized MacBook. When is it 0.35 cm thick and when is it 1.31 cm, for example when increasing storage or changing CPU?
The case is tapered.



It's thicker at the back than the front.

Pic from Apple Store - Macbook