Hi Pilou - If you look at the original model, it has a kind of "steep"-ness to it, that is the type of thing generated by extrude.

Shell will produce something like this:



That kind of has movement in many different directions, the new surface is along the surface normal of the original shape and not just in one single direction.

Extrude moves only in one single direction, that's more like the original shape.

- Michael