One explanation is missing: the angle of the edge. If the blade edge has been sharpened to 20 degree angle, it has the same 20 degree cutting angle when used d

One explanation is missing: the angle of the edge. If the blade edge has been sharpened to 20 degree angle, it has the same 20 degree cutting angle when used diagonally. The effective cutting angle decreases in the direction of the cut when you rotate it. The smaller the angle, the sharper the knife.