It's a line of sight kind of thing where they relieve the chamber on the far side if you just lean the head with the valve out you'll see it matches the shape of the far side seat.
If only we could think of everything all at once we would remember that the General consensus is open chamber heads tend to have better numbers, not all but that's because everything's intertwined in its design. Always exceptions to the rule but pretty much you know what I'm saying