Most of the 1 kb ladders I have used are made by the company producing them. That is they use specific, cut plasmids or purified DNA created in their facilities to make the bands in their ladder.
"A number of proprietary plasmids are digested to completion with appropriate restriction enzymes to yield 10 bands suitable for use as molecular weight standards for agarose gel electrophoresis. The digested DNA includes fragments ranging from 0.5-10.0 kilobases (kb). The 3.0 kb fragment has increased intensity to serve as a reference band. The approximate mass of DNA in each of the bands is provided (assuming a 0.5 μg load) for approximating the mass of DNA in comparably intense samples of similar size."