BP PLC and other big oil companies based their plans for responding to a big oil spill in the Gulf of Mexico on U.S. government projections that gave very low odds of oil hitting shore, even in the case of a spill much larger than the current one.

The government models, which oil companies are required to use but have not been updated since 2004, assumed that most of the oil would rapidly evaporate or get broken up by waves or weather. In the weeks since the Deepwater Horizon caught fire and sank, real life has proven these...