You will see black bands when the aspect ratio of the recorded video material does not match the screen aspect ratio of the iPad screen. As is, you are seeing the recorded video exactly as intended.
Your iPad screen aspect ratio (the ratio of width versus hight) is approximately 4:3 - whereas widescreen movie material is often 16:9 (i.e., much wider than it is tall), resulting in a black band above and below the visible image. If the 16:9 material is expanded to fill the screen, one of two things will happen…
a) If the height is simply increased to remove the bands without also widening the screen, this will introduce image distortion; vertical objects will appear artificially tall and thin.
b) If instead the hight and width is increased to remove the bands without introducing image distortion, the left and right edges will be cropped from view. In this situation you have effectively zoomed-in to the visible image - and hidden the edges.
I hope this explanation is helpful with understanding both what you see - and why.