Alloying pure lead, monotype, etc., in addition to adding 2% for castability with other metals. There's more use for tin than just what you're experienced with.
I mixed up a small batch of 10:1 by weight with pewter instead of pure tin just to see what I'd get. Weight was where it should be, and BHN was 11.5 where it should be. Unless, maybe, the Lee tester is total garbage, then I don't know.......
10:1...?? by who's testing, Lyman? Who ? How do you know who has the right mixture to come up with the right hardness? From what I heard Lyman isn't correct.... According to thier chart my alloy is in between electrotype and 1:1 lead/Lino.. not WW. You said yourself that not all WW's are the same, and I tend to agree, now. Brand, manufacture, mixture, etc.. who knows what I have, I just know what do have is 13 BHN. It doesn't lead my barrel with my 44 mag with full house loads, without GC's. Kieth never used checks with 11 BHN.
You can't get it that the Lee Tester isn't pure garbage. So yeah you don't know, and surprisingly.. I'm surprised you said you don't know, for a change.
I just talked to a engineering company in Canada a little while ago.... and the Spring in the Lee is precalculated along with the indenter ball for a particular material, in this case lead/alloy. You are not giving credit where credit is due. You need to stop overthinking this and just move on. If your casts are accurate, and fit your cylinder throats and groove diameter (and doesn't lead your barrel) and performs on game, then call it a day, otherwise continue to chase your tail with your doubt.