Create Perfectly Clear Cocktail Ice With Directional Freezing

Ice does not form top down.  Like any other form of energy transfer, it takes the path of least resistance first. Standard ice cube trays are designed to have air flow around the entire cube promoting quicker freezing. So you would be the top frozen first (no insulation) , then it will go around the sides and bottom, than the middle fills in.  The cooler forces the cold air to come from the top and freeze from that direction as much as possible.