I think the variation you are seeing is likely just variations from batch to batch. All three of us are (were) using Horween Horsebutt and they really only do 3 tannages on that, Cordovan, Chromexcel and Veg tanned. Every batch I have ever gotten from them, while similar, felt or looked a bit different each time.
I agree the differences are subtle, the piece I got from Neil is the slickest but not by much.