Let’s look at a simple example…
You make a 1m³ cube by PushPulling a 1m square face up 1m.
You group it.
It is a Solid in Entity Info reporting 1m³.
It is exactly that a solid block of ‘stuff’.
There is no ‘void’ in the simple cube.
If you want it to be a hollow ‘box’ then it needs to have [thin] walls enclosing an empty volume.
If you want that ‘void’ volume to have something in it, you must make a separate solid group that fills it.