The reason is that this is a large concrete bowl that has thick walls and a thick bottom. Using a higher proportion of sand will give it added durability.
Once each cement mixture is the consistency of a wet mud pie, pour them both into the large outer bowl mold and then mix them together.
Once the large concrete bowl has cured, remove it from the mold.
The top edge of the concrete bowl is probably a bit rough, so go ahead and sand it. I sanded both the flat edge and the sharper corners of the edges.