That is why you see numbered steels because they represent a certain composition. However, mechanical properties can be greatly influenced by how they are treated during manufacturing. Steels can be quenched in water to give one value of hardness, whereas they can be quenched in oil to give a different value for hardness. Most of the mechanical properties are influence on what phase of material is allowed to develop as the steel is made. Quenching is just one way to influence phases and grains of a piece of material.
Heat treating and annealing are just a couple of other ways to work a metal.
Hope this helped. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.