I see it partly as mdi(post #17)in the reference towards the difference between swaged,pure for BP,and sorta... anything else must be "hard cast". One other little addition is in plumber speak. And that's a reference back when lead pipes were in use... soft lead vs hard. The latter being used as wiping lead on joints. Which was a much richer(harder) alloy.

So I guess it comes from a cpl sources,thrown together and hard cast is how it ended up. It's not something to get wrapped around the axle about IMO. Sorta like clip vs magazine?

But one thing's for sure,high BHN numbers are most often overrated. To the point,I think the goal is how low a number can you utilize. Good luck with your question/research.