In a crab, you are correcting the HEADING to maintain the TRACK down the runway. That is -- to keep the airplane tracking down the runway centerline you have adjusted the direction that the nose is pointing, to maintain that track. The plane is not weathervaning, you are pointing it in that direction, whether you realize it or not. If your HEADING is straight down the runway centerline in a cross wind, the TRACK will fall off downwind, even though the plane is pointed where you think you want it to go.