Usually the variations come from who they're counting. The 20 million range is direct military casualties. i.e. just the people that were in armies. The 50 million range includes all the soldiers plus the civilians that were killed by military operations. i.e. civilians killed in fire bombing of cities. The highest range, the 70 million range, includes everyone, the soldiers the civilians killed in battles, and everyone that was killed due to famines and disease outbreaks that were caused by the war.