Older lead will read harder than newer smelted lead.
Could be that your casting and smelting temp and cooling speeds were different.
I dont bother checking my ingots for hardness. mainly cause they are straight WW ingots and I know it. I run my melting pot at basically the same temp all the time and the boolits run a pretty consistant hardness.
If you want harder boolits, just water drop or heat treat them. They will come out as hard as you need them even for max loads. No need to add other expensive metals. This is what I do anyway.