The reason they became known as "matte" paintings is because one of the ways to frame a picture you hang on your wall is to use a matte - a colored piece of cardboard that surrounds the picture that allows you to put a smaller picture into a larger frame instead of putting it in a frame the exact size of the picture. The FX paintings became known as "matte" paintings because they work the same way - surrounding the filmed element to create the illusion that the total image is bigger than what was on the original film.