When you're in gear, the engine is kept moving by the momentum of the car. When you switch into neutral and break the connection between the movement of the car and the engine, the vehicle has to use fuel to keep the engine moving and not stall. Yes, resistance will cause you to slow down quicker in gear than in neutral, but during that time you should be using basically no fuel compared to using the same amount of fuel that your vehicle would be using while idling if you're in neutral.
Basically, while you're coasting in gear, you're using the car's momentum to run the engine rather than fuel. When you switch to neutral, the car's momentum keeps the car moving forward, but the engine needs fuel to keep going.
EDIT: Popular Mechanics has an article on it http://www.popularmechanics.com/cars/hybrid-electric/a5977/c...