I think it's saying if the two panes were sealed without a hole at ambient pressure when the plane is manufactured then the outer pane at elevation would have ~1atm of pressure pushing out on it and the middle pane would have ~1atm pushing in on it. With the hole the outer pane only has cabin pressure pushing out on it, which is ~75% of 1atm.
If "what to make X" is your design choice, you would not want to have it always pegged at 1 atm, as it would be if it was/could be completely sealed at sea level. It's simplest to have it equalize with the cabin pressure.
