Why is basement roof slab reinforcement different from floor slab reinforcement

Floor slabs are loaded from the top to the bottom, and these forces eventually pass through the basement all the way to the foundation. The basement slab needs to carry the loads from the upper floors, so it needs stronger reinforcement. And the higher up the floor slab is subjected to less force and doesn't need as much reinforcing strength. Therefore, most of the basement roof slabs have a two-layer bi-directional arrangement of reinforcement.