Upon analysis of casualty figures from the world wars one will find incredible discrepancies in terms of the range of supposed deaths. Why are there so many "estimates"? For example the wiki for the battle of Passchendaele lists the casualty count as 200,000–448,614, a MASSIVE gap (I understand that Wikipedia isn't the most credible of sources but this issue isn't unique to that website). Wouldn't soldiers be registered at home? Wouldn't families be notified of their child's death and is that not documented? Why are there be so many forgotten soldiers?
Basically, "casualty" is a vague term that has a lot of factors associated with calculating it. There are military and civilian causalities. There are soldiers and civilians killed, wounded, and missing when a battle occurs that are all part of the casualty estimate. These all function as separate variables that must be included in casualty estimates. Each variable has a high and low range estimate and combing all of the variables leads to a wide range for casualty estimates. Further compounding this, from any given battle there are also deaths from disease and other causes not explicitly related to wounds suffered during a battle.