re: Something smells here...
I certainly agree that more studies are needed.
Iraq is a chaotic and dangerous place, and there may be reasons that families would under-report violent deaths. I think you would agree that the US military and administration (and the Iraqi government*) also might be motivated to under-report. In both cases, butt-saving is involved, but the latter is IMO not particularly noble.
* One might cynically include the Iraqi gov't as part of the US adminstration