Look at the area OPPOSITE the spark plug side of the chamber in the second picture. See how it is "open" compared to the same area in the first picture? That flat area of deck surface in the first pic makes quench possible. It also lessens the chamber volume and brings compression up, all other things equal. Again, first is closed, second is open. Look at the red X in each picture. It's in the same spot on each head. Now, look at the difference in that area on each head. See the difference?
