Because at least 70% of the leak oil turns into gases at surface temp.
According to Wikipedia "Petroleum", under the surface temp and pressure, the lighter hydrocarbons, such as methane, propane, ethane, butane occur as gases, while the heavier hydrocarbons, such as pentane, are in the form of liquids or solids.
And the proportion of lighter hydrocarbon in the petroleum mixture ranges from as much as 97% by weight in lighter oils to as little as 50% in heavier oils.
Given the gulf oil well produces "lighter crude", the 70+% of leaked oil turn into gases at ocean surface is a very reasonable estimate.
What happens to the oil that is in the water because of the dispersants? It never gets to the warm surface.
When you use a dispersant (detergent) in greasy dish water, the grease is still suspended in the water.
That is not good news as all of the chemicals such as Benzene that are found in light sweet crude are carcinogens and much more toxic in much lower exposure rates than heavy crude.
BP will be fighting cancer lawsuits decades from now. Just like Owens Corning.
I am searching for truth.
If it is true, then the potential environmental damage is over-estimated. In the oil spill in Alaska, the oil leaked is heavy crude. But this time it is light-sweet crude.
50% of liquids and solids v.s. as low as 3% of liquids and solids in this current case could make a huge difference.
If you decide to be blind to the truth, then ....