I wonder if it could possibly be because there are different types of witches. In South America there are people who sacrafice chickens and pigs and read their entrails. Some countries in Africa still have witch doctors who remove the hearts of goats; they are the only ones who are allowed to sift through the organs and read them. I suppose that human sacrafice isn't such a leap either since there is evidence of human sacrifices all throughout MezoAmerica and Egypt and several other places. Perhaps people, not knowing better, lump all witches into one group and consider them to all practice the same since they all share the same name.