Because the higher you can inflate the numbers, the more international (U.S) $$$ you can expect.
The Haitians would have been inclined to inflate numbers to increase aid, as you suggest. On the flip side, the Chinese would tend to deflate numbers as a matter of pride.
This is true but the ones doing it and getting the money are the aid agencies. There was a transparency international report that 938 million dollars raised for tsunami simply disappeared and the culprits were the aid agencies.