Why does my game have letterboxes on the side?

I hit build and build and run for my 2D game in unity but when I play it there are letterboxes on the side. It happens in the built in player too. I have the aspect ratio set to 16:9 which I’m pretty sure is wide-screen but it still shows the letterboxes. Why is this happening?
BTW I’m using a 1366 x 768 display I think

It turned out that it just does that in the editor. However, in the real game (build and run) I simply set the default screen resolution to 1366 * 768 instead of the automatic default of 1024 * 768. I think the option for default screen res was in build settings.