I guess that you tried the "letterbox scale"
This has to do with the dimensions/ratio of your project. Construct won't stretch a square project (let's say 512,512) in to a wide screen rectangular shape like 1280,720 (16:9).
It will stretch and fill the height (512px -> 720px) and it will leave 256 pixels of black borders to the left and right of the project (1280-768=512/2=256), in order to keep the square proportions.
If you try "scale outer", construct will fill the black areas with content from your layout, some times though this isn't desirable because it messes up with the framing of your project, and by framing I mean what things you allow the user to see or not with the window dimensions.