Recording at a resolution with a non-standard aspect ratio (e.g. 1280x800 @ 16:10) can add letterboxing to your video.
To correct the problem for future recordings, try following these steps:
Open the Screen Recorder app.
Click Size.
Select 720p.
Resize the content you're capturing to fit within the 720p recorder frame.
This will avoid the addition of letterboxing in your videos because 720p has a 16:9 aspect ratio. Using any other resolution with a 16:9 aspect ratio (e.g. 1080p) will also avoid letterboxing.