1. Don't have a front-facing camera on your laptop. Your actual laptop screen can be as close to the edge of the laptop case as technology will allow, with no bevel.
2. Have a front-facing camera on your laptop, with no bevel. Your actual laptop screen is now square, but has to be 5mm shorter now. There's a 5mm strip at the top of your laptop that can't be used for anything.
3. Have a front-facing camera on your laptop, with a bevel. Now the actual laptop screen has a "dead space" in middle at the top, but can again be as close to the edge of the laptop case as technology will allow. There's a narrow strip at the top that can't be used for anything, but the area to the left and right of the bevel can.
(A number 4 would be to somehow have a front-facing camera that operates without needing to displace screen area. Not clear how this would work without some complicated mechanism to extend the camera out the top of the screen, which would come with its own problems.)
Now, the vast majority of the time, you're going to be using your Mac in a windowed environment, with menus on the left, indicators on the right, and absolutely nothing in the middle.
In the case of #2, this menu bar has to take real estate from the top of your shorter screen, meaning that your windows are all 5mm shorter. #3 allows the menu bar and indicators to take up that space which on #2 is completely dead, freeing up extra space for your actual applications.
And the key thing is this: For modern full-screen games, in #3, you (apparently) can't use the areas to the side of the bezel; but this is the same situation you're in in #2.
IOW, as another commenter has said: The bevel design doesn't take away screen in the middle; it adds screen space on the side of the bevel.
That said, the API here seems obviously mad: What's the point of giving you a resolution if it's going to silently resize it behind your back? It should either give you the resolution which it won't resize, or throw an error when you try to make one higher.