The variability within a 4140 steel from one heat to the next is sufficient to cause variations in hardness if the same procedure is used every time. Variations from the top of the ingot to the bottom can do the same. This is due to the natural segregation of the alloys during solidification. Heat treaters usually demand the mill test reports on steel when large tonnages are involved. They generally make subtle adjustments to their process based on the actual chemistry. Even then, not every piece will be the exact same hardness. When I hear that someone has a process that works, I say stick with it. You have the major variables under control and small variations, while having a predictable effect, are not affecting the performance of the product. That's what good engineering, and smithing is all about.