the current trajectories taken are all shortest path trajectories. but shortest path doesn't necessarily mean best due to radar early warning, missile defense, presence of ships, etc.
launching 'backwards' around the world i.e. launching in the great circle across central Asia towards east coast and overflying Mediterranean might be better against a certain adversary missile defense than launching across polar path alone. this would require a longer range than the polar path will provide, hence a reduced payload to accommodate this.