Frame profiles and the curvatures of bend have to do with the equipment used to form the frame pieces. Large stocking manufacturers use roll forming equipment to form their standard jamb width material (such as 5⅞”). Custom manufacturers use press brakes to form all of their material. Although the jamb widths are the same with either method, the curvature of the bends of the profile vary from roll forming to press braking, and from press brake to press brake. Therefore, the material will not make a perfect match when put together. Good installers, however, are able to make material from different manufacturers match on the job, if the pieces are field welded.
Another problem, inherent in knocked down frames, is the system used to fasten the corners of the frames together during installation. One manufacturer may use corner clips on the jambs, that lock into holes in the heads, whereas another manufacturer may reverse this and use holes in the jambs that lock into corner clips in the heads.