On the first scene’s layout settings’ (the 1200px to 1600px) width is set to 1240px, and the scale 100% is checked. The result on lates 13inch mac pro (both safari & chrome) is that the elements are not fullwidth.
The way I would handle this specific layout is to make the background image scale to fit the available space (pinned and scaled on all sides with ‘Expand to Fill’, but make your text box and text box background only set to pin to the left, top, and right so that it stays in the center top of the page.
To get your background image to fit the available space, adjust your scene size to match the image size.