Parent & Child Table key fields

In the gint std us files, the Library, data template and subsequent project database files;

I understand that the Soil Components tab is a child of the Lithology tab, and

that the PointID and Depth are the key fields for the Lithology tab and that those fields are subsequently the key fields for the Soil Components tab.

What I don't understand is why is the Soil Components tab also pulling the Bottom depth value from the Lithogoly tab as well the Depth?

The Bottom value is not a key field and I am not seeing where the Soil Components tab is being told to pull this value from the parent table.

Thanks,

Brian L.