Why is Novak Djokovic the world number 1?

Why has he overtaken Rafael Nadal as the world number one? I looked at the ATP schedule at it says that 2000 points are awarded to the winner's of Grand Slams. And before wimbledon, rafael nadal was ahead of djokovic in points. So how does djokovic over take nadal even if nadal wins wimbledon? Why doesn't Nadal get any points for winning wimbledon?