While I don't like the look of it myself, I'll hazard a guess as to the design thinking behind it: notifications/menu bar can now live either side of the notch without taking up 'content' space. So the screen either side of the notch is for the notifications bar/menu bar/I-can't-remember-the-android-term-for-it and the total space matches the space on the bottom of the phone, leaving a centered amount of screen space for 'content'. Because it's not most full-screen experiences are ever going to use the notch area.