Hopefully not too late for this and if I missed it please forgive me...
TH level and how it's utilized is a big factor as well. IE: TH4 pulled stones only, THIV on stones and all mobs, etc. While THIV is a lot more common now, I don't think it's a safe assumption that all shells have it, although most I'd assume do. When talking about specific drop rates, I believe it needed in the discussion as to not skew results.