While wider tires do have less rolling resistence, that only applies to a point. The limit is around true 28s and then rolling resistence starts to increase again. In the case of Conti 4000, the 28 is a few watts slower. The same applies generally to other manufactures although I don't have that data in front of me right now. There's also an aero penalty with wider tires.

When people switch to 28s, it's for reasons other than speed.