What causes the weird shading when you smooth shade a cube?

Everyone knows the infamous ugly shading when you smooth shade a cube (or other sharp structure):

enter image description here

My question is not how to fix it. My question is what causes the cube to be like this? I know “the normals are averaged”, but what exactly it means? Why doesn’t it look like a super smooth sphere, for example? What causes these separations between bright and dark sides?

enter image description here

It is just already what you know, it averages the normals to smooth the lightning.
If you want faces smoothed only within specific angles you can activate “Auto smooth”