Plumer explains:

The story goes like this: Back in 1992, Congress passed a law requiring all federal agencies to buy alternative-fuel vehicles for 75 percent of their light-duty fleet. The catch was that, while the agencies had to buy the cars, they didn't actually have to use the alternative fuel. So a lot of agencies ended up purchasing cars that could run on propane, compressed natural gas, or E85 (an 85 percent ethanol, 15 percent gasoline blend), and them shipped them to areas that didn't actually have any alternative fueling stationsthe infrastructure just wasn't in place. Fewer than 0.1 percent of fueling stations in the United States even offer E85. That meant most flex-fuel cars were running on plain old gasoline, and, since these vehicles generally have larger-than-average engines, they actually end up using more oil and emitting more carbon dioxide. The Postal Service used 1.5 million additional gallons of gas last year because only 1 percent of its 37,000 flex-fuel vans were actually running on ethanol.