The drone ship and the rocket work independently. The drone ship uses GPS to position itself as closely as it can to a fixed position in the moving sea, and the

The drone ship and the rocket work independently. The drone ship uses GPS to position itself as closely as it can to a fixed position in the moving sea, and the rocket blindly lands at the position where the drone ship ought to be. So sometimes it will land in the middle, sometimes closer to the edges, depending on how steady the water is. The drone ship is huge so there is no need to land precisely in the center.