citizen, all frames on a DVD must be 720x480, regardless of the picture information. Commercial movies that are cropped/resized to 1.78:1 (16x9) will fill the entire screen, but 2.35:1 films like Star Wars require black bars at the top and bottom of the frame, because the film is far wider compared to its height than the actual DVD frame. While it is difficult to see, ALL DVDs with a ratio greater than 2.35:1 will have black bars at the top and bottom of the frame - even on a widescreen TV - and if you play it back on a 4x3 TV, the player will generate bars that go above and below those bars as well.

In your case, the bars you placed in your frame were not quite black, so the distinction is easy to see. On most commercial DVDs, the black bars in the frame are very black, and very close to the black generated by the player, but if you look really close, and adjust your TV's brightness/contrast, you will see a line dilineating between the black bars in the DVD frame and those generated by the player.