It's very possible for them to have served overseas. Older men did serve overseas, even if most of them were young.

Did you ever come across any reasoning for this?

WWII is indeed a vast subject. I've studied it for two years now and I'm still learning things.

In your research did you ever come across a documentary called World at War? I'm wondering how reliable an information source it is. Although apparently (as it was filmed in the 70s) it has interviews with several key military figures from the war.