Movie theaters originally showed movies in 4:3. Then TV came along, and to compete, movie studios began making movies in "widescreen." There are different aspect ratios, but generally they are a bit wider than 16:9. Many were produced with TV in mind, so that the edges could be clipped off (or pan and scan used) will little adverse effect on following the movie. This let them more easily sell an older movie to TV. Anyway, an original aspect film will often have black bars on the top and bottom when shown in HD on a widescreen TV.
I have read that varying types and quality of film has been used over the years to make movies and TV productions. With 35mm, the inherent resolution is 2 to 4 times the HD resolution of 1080. So these can be converted rather easily to HD. By cropping the tops and bottoms, and perhaps with a bit of stretch, they may make it to 16:9 or come close. I've seen productions in widescreen where they had columns on either side of the show. This produced a narrow screen actual image, but at high quality, and the side columns gave the effect of watching in a movie theater.
Of course, they can try to interpolate and increase the resolution and call it HD, but the result is disappointing. They do this with old taped shows.
