Why is the reinforcement of basement roof different from that of floor slab?

The floor is stressed from top to bottom, and these forces will eventually pass through the basement to the foundation. The basement roof needs to bear the load of the upper floor, so the reinforcement is required to be strong. But the higher the floor, the smaller the stress, and the greater the strength of reinforcement is unnecessary. Therefore, most of the basement roof is reinforced by double layers and two ways.