The letterbox Integer scale, as his name implies, only scale at integer/integer Values (like x1/4, x1/3, x1/2, x1, x2, x3) and will hide everything else.
The letterbox Scale mode will scale the game correctly, however their will still be black bars on sides to compensate the difference of aspect ratio (if there is a difference), but could be enough for you maybe.
The Scale Inner and Scale Outer modes are filling the screen entirely, but requires a carreful and dynamic designing.