From The NYTimes today:
About the Oil Slick Areas Shown on the Map
The “probable extent” of the oil slick is an estimate by the National Oceanic and Atmospheric Administration of where oil is mostly likely to go based on wind and ocean current forecasts, as well as analysis of aerial photography and satellite imagery. The “observed extent” show areas where oil was visible on the surface of the water during aerial surveys of the Gulf. The observed extents are not available every day. The extents may vary widely from day to day because of changes in wind patterns and ocean currents.