"I understand that 80% of the people followed the order to evacuate. That means that there were 250,000 to 300,000 people who were left in New Orleans"
Please don't forget that these are tourist areas, and the number of people present could exceed the population figures by a great deal. I'm sure that many, many people evacuated from the coastal areas, but many also chose to stay based on whatever security they felt based on past events.