[Chris Goss]

I am not an expert on such matters but my personal opinion matches that with Jon. I agree that the US strived to bring their dead home or, as in the case of Omaha Beach, the cemetery became American soil in perpetuity.
The scale of British losses in WW1 were massive and many were grouped together to be buried close to where they fell (only for the cemetery to be obliterated in subsequent offensives-if you look colosely at the earth around the graves at say Sanctuary Wood, you will see many fragments of bone)
WW2 was a different matter again-those who were buried where they fell (or crashed) were identified and later moved to large cemteries-Bad Tolz, Hannover, Reichswald