Recording at a resolution with a non-standard aspect ratio (like 1280x800 @ 16:10 for example) can add letter-boxing to your video.
To correct the problem for future recordings, try following these steps:
1) Open the Recorder app and click 'Size', then select '720p'.
2) Resize the content you're capturing to fit within the 720p sized Recorder Frame.
When you create a new recording that way, you should no longer have letter-boxing in the saved video file. (You can record at '1080p' too, if you have that option available).