Similar to [0.6.6.5] Crossbow Scope Does Not Adjust to Resolution
This time, you can notice it, on a widescreen monitor (21:9).
One possible solution, as suggested by this mod for Left 4 Dead is to use different overlays for other formats :
Another suggestion, this time from me (From my previous experience, making custom scopes overlay on Old-School Call of Duty games), is to make the scope overlay texture, as square in itself, covering not the entire screen (So, for example, a texture of 1980·1080, the scope alpha at the center, and the rest being entirely dark) to a square (They were using a 1024x square texture, but you could upscale it to 2048x) that will be placed at the center of the screen. And since the rest is just pure black, simply fill the remaining screen with the color black.
This should even kind of fix an unfair advantage to user with an ultra-widescreen monitor, which currently have a larger zoomed view due to the way the 16:9 is stretched out, using a square at the center, should display the same area equally for every resolution.