Hazelnuts39,
Setting cost index to zero should not get you flying at green dot. You should be higher by 15-20 knots out more depending on altitude and winds.
If you look at a typical L/D curve for jet aircraft, it is quite flat close to L/D max. Flying at GD+20 would cost the same than flying at GD. Maybe not the same exactly, but no noticeable difference, so Airbus chooses some speed higher than GD.