The official death toll is only a fraction of the US troops that have died in and around Iraq. If their death does not involve a projectile of some sort it is a non combat death and is not counted.
For example if a hummvee drives into the river to avoid an explosion and all on board drown, they are not included in the official death toll. If they contract a fatal disease (engineered or natural) they are not counted.
The actual total is guarded. The actual total is....
sorry but Woe be to the journalist who does not heed the advice of the new and improved special office of the Pentagon.