Given that you're asking about the Wikipedia article, it seems relevant to point out that Wiki has a dedicated explanation of its own reporting of WW2 mortality. Both in the text of the article and if you look at the page history, it all comes down to the same thing: whether or not civilian deaths from famine and disease are counted, and more specifically, which civilian populations are included.

I hope someone who knows the politics and historiography of particular counting methods will come along. But in the meantime, hopefully the explanation of what Wikipedia is doing helps clear things up.