Ground material for closed space

[Higuchi]

Hi guys,

I'm struggling to render the completely closed space from outside. As you can see it, I tested two option below. By the way, environmental light is totally turned off.
1. Disappear the ceiling and near wall
2. Apply "Ground" material for the ceiling and near wall

My question is about the difference. According to my understanding, both are should be same, because "Ground" would work as much as disappeared model other than the shadow. But, they look different, could you tell me what happens and more details about "Ground" material?

Thank you,

[Higuchi]

Additionally, even if I adjust some parameters such as Specular Color and Refraction Index, the appearance was not improved at all.
https://manual.keyshot.com/manual/materials/material-types/special-material/ground/

I just want to properly calculate the reflection from not only appeared wall and ceiling but disappeared ones.