The aim composition as stated by cloa is an optimized composition, usually generated by the manufacturers material engineers. Since melting and casting of materials, or blending of raw materials isn't always an exact science there has to be some leeway from optimum to acceptable. Hence, composition ranges. In a foundry situation, for example, there are literally hundreds of things that can influence the final results for a heat of steel. In a perfect world what went in to the pot comes out all blended nicely together, this isn't a perfect world.