The mesh is low poly and it has a nice topology, and the normals are all consistent; all is smoothed.
It’s not the first time that I obtain this (I am just in training) so now I want to understand how to correct it and why it happens.
You might want to do a forum search for “terminator artefact”, as this has been asked and answered a gazillion times before…
In a nutshell: Smooth shading is a trick that can only go so far. When high-contrast lighting meets low-poly geometry, that trick will fall short and the shadows will terminate in an incorrect way - that’s an issue very many unbiased render engines suffer from. So, either use a larger light source (to “soften” the shadows) or increase the density of the mesh (higher level of subdivision, for example).
Oh thanks I didn’t knew that (nor that it was called “terminator artefact”). 
I’ll search this.