Well, maybe indeed you are right and the Americans had different administrative policies than the Germans regarding KIAs and MIAs (actually I have read that even the USMC and the US Army had different administrative policy when it comes to reporting WIA casualties - "definition" of a Wounded In Action was slightly different in the USMC than in the US Army, and as the result the USMC usually reported more wounded per each killed than the US Army).
It can be confusing because some historian might take a look on these figures and say: "What the heck, why did the US Marines always have a higher percentage of wounded among their casualties than the US Army? Oh, maybe the Marines were better in hiding from enemy fire?!". And this is wrong - the explanation is much more trivial - different administrative policies, different methods of judging who is "wounded" and who is just "contused" or "injured" (less serious "damage").
I only wrote what I know that was the case in the German army.
And if you aren't satisfied with the Poland 1939 example given above, then let's take a look on casualty data from the Western Campaign of 1940.
In July 1940 (so one month after the end of the campaign) Germans reported casualties of the entire Wehrmacht (I think that these are numbers for the entire Wehrmacht, not just for ground forces) in the Western Campaign as 27,074 dead and 18,384 missing (in total 45,458).
By comparison a report compiled 4 years later - in 1944 - gives revised casualty figures from this campaign - and they are 45,429 dead and 630 still missing for Heer (ground forces) alone. Add to this 3,756 dead or missing in Luftwaffe and 289 dead or missing in Kriegsmarine. In total around 50,000.
So as you can see not only vast majority of those reported missing one month after the campaign (July 1940) were later revised as in fact dead guys (no surprise, considering that in July there were already no German POWs in French captivity), but also the overall number of casualties increased (perhaps due to incompleteness or inaccuracy of those early reports, as well as probably due to the fact that some of the wounded died of their wounds later).
The same happened in case of casualty reports from Poland 1939, as I already wrote before. All in all in Westfeldzug 1940 Germans lost ca. 50,000 dead (including 4,000 from Luftwaffe and Kriegsmarine), while in the Polenfeldzug 1939 - ca. 20,000 dead. In fact I prefer such "round" numbers, because exact numbers are never completely accurate, so there is no reason to cite them and create the impression of total accuracy where there is no total accuracy.
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Another interesting case to show you how important is the context when you want to draw conclusions from casualty figures.
During the last months of combats in Tunisia in WW2, Germans and Italians reported the following casualties:
- 0 (zero) KIA
- 5,000 WIA
- 180,000 MIA
This was due to the fact that this month ended with destruction / capitulation of the Axis forces in Africa.
Those 5,000 WIAs, are only those WIAs, who were evacuated from Africa via the Mediterranean Sea. No KIAs were reported as you can see.
On the other hand, among the 180,000 MIAs are many wounded guys (who got captured by the Allies), as well as many dead (KIA) guys.
Majority of the MIAs are, however, captured POWs, because the Axis forces in Tunisia surrendered en masse during that month.
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