They are constantly stopping, turning and changing directions using one edge or the other. The hollow grind makes it easier for the edges to cut into the ice. A circular grinding stone, that is rounded on the edge, rotates rapidly cutting a hollow grind into the bottom of the skate blade. The center of the blade needs to travel over the center of the stone from the toe to the back of the blade so the edges are level the entire length of the blade. Coin is perpendicular to the blade.
Edges are level.
Edges are not level. Have these skates resharpened. After a pair of skates are sharpened, check to be sure the edges are level. This can be done by holding the skate upside down and placing a coin or metal washer across the blade. The coin should be perpendicular to the blade.