Because they're decontextualized. I'd posit that in the context of small organizations most of those murders are a product of deliberate and known risk. Given that, one could liken it to mutual combat at the ethical and social levels. That isn't the whole scope of the matter but the aggregate data would, I suspect, reflect a considerably different picture if it was investigated at such a resolution. Of particular interest would be infidelity and how that fits into the context of a given social order.