Question about the perception distance along the z axis.

[Honey-ashead]

It is well known that the ego frame is with reference to the center of the real axle (approximately where the IMU is at) projected to the ground. However, I noticed that the perception distance along the axis z in FastBEV is between -5m3m (point cloud range) and the occupancy space along the axis z is define within nearly -4.75m1.25m, where the heights w.r.t the center of four layers of the defined occupancy space are -4,-2.5,-1,0.5m separately. In this case, the occupancy can only focus on only a little space above the car, while it concentrates more on huge space below the car, which makes me confused. Can you give me some tips?