I think you are right, it is the finder app that behaves a little differently, which I’ve always assumed is because you typically need to drag files in and out

I think you are right, it is the finder app that behaves a little differently, which I’ve always assumed is because you typically need to drag files in and out of the finder so it leaves a gap to make that easy, but you can still make it full screen if you want. I’ve never noticed this with any other apps.