58-60 is a range of hardness for the hardened area which may or may not include the whole blade as Adamlau stated. These are typically general specs that are put out by manufacturers to measure hardness given the appropriate heat treating process. Since they don't test every piece of hardened meal that they produce and since there may be some error in the heating, quenching and tempering processes, a range is used to specify hardness instead of a discrete value. The typical spine on a differentially heat treated blade should not be anywhere near a value of RC58 - to do so otherwise would invalidate the reasoning for differential heat treating in the first place (remove brittleness and restore resiliency to the spine).