Because the government will want to keep the numbers as low as possible... they do it all the time with all sorts of illnesses.. like HIV/Aids... the patient has the disease and dies from pneumonia (caused by a weakened immune system) the cause of death: pneumonia not HIV/Aids so we keep the "official" numbers down and it makes the government look good.
Also if people are declared missing. They don't count for any compensation.
Uhh ok. So instead of (insert however many thousand) people dying in Hurricane Katrina it'll be (insert however many thousand) minus maybe a couple of hundred. Gotcha. Makes perfect sense.