Notice that a shadow is adjacent to the bottom face. It always moves with the bottom face to provide a constant, almost subliminal, indication of the Up direction of the model.
For most models, the sense of “up” is inherent in the model, or is not important. However, for some models and modeling situations, such as running a gravity-influenced motion simulation, a clear and constant sense of up and down may be useful or important. The shadow does not display when the Bottom face is visible.