Yossam what i understand is this:
The shape shader comes always where the real 0,0,0 position is on the Planet .
It isn't important if there is any displacement or not.
When you use displacement your planets altitude changes but the shape shader is still on the undisplaced 0,0,0 coordinates like before.
Not sure if this makes sense but it looks confusing yes.
Your shape shader is under the ground basically
now.So you have to use the displaced terrain altitude.