Many films are shot in a lower aspect ratio than the ultimate showing is supposed to be. SOmetimes it is just because the film frame is (usually) 4:3 or sometimes because the director wants to have extra material to fill the TV screen instead of black bars, and not crop the sides of the picture using pan-and-scan. In the theater the projectionist is supposed to cover up this extra material by adjusting mattes in the projector. (On some films the camera has mattes in place so no extra material above and below is included and black bars are put directly on the film. THe projectionist should redundantly cover these black areas with mattes otherwise scratches in the film will show through and project above and below the picture.)
Sometimes the extra material above and below the widescreen picture meant to be seen in the theater is present but not usable. There might be bloopers there. The special effects might not stretch that far. So for some scenes of the full frame version the picture has to be zoomed in and then pan-and-scanned, thus many videophiles don't trust the quality of this method of production.
On the other side of the DVD with the wide screen version the extra material above and below is blacked out. I would not be surprised if the method of doing this blacking out is imperfect so a faint residue of the extra material above and below showed through on what should be the black bars.
Video hints:
http://members.aol.com/ajaynejr/video.htm
