TomD
It used to be said that it was hard to make use of more than 1% carbon, which is why a lot of serious cutting tools are made of steel with 90 points. The stuff that can be going on it often a lot more complex that simple rules. You can certainly get stuff really hard with less.

One thing that might be worth considering is whether steels with the same chemical composition are in fact the same steels. Could there not be differences in steels in which assembly was done differently, in different order, with different conditions.