On the 'leather seat' option the sides and top are not real leather ('Ambience') but the middle bit is classed as leather.
Actually, the only part of the seat that is not leather is the head restraints, which are the same plastic regardless of upholstery choice.
The 'Ambience' on 500 refers to the colour of the head restraints, steering wheel, audio controls and heater controls, not the material that anything is made from.
On 500 Sport there are 4 upholstery choices:
Grey-Black Cloth
Grey-Brown Cloth
Black Leather
Red Leather
All come with Black Ambience.