The reason the high expansion ratio example is less effected by barrel shortening is that by the time the bullet reaches the end of a long barrel it has already run out of steam, so to speak. If the muzzle pressure is only 5000 psi less performance will be lost than if it is 15000 psi. The low charge/high expansion example will be more efficient but not necessarily more effective than a round with high charge/projectile ratio. A lot of shooters are less concerned by efficiency than by effect. They are willing to burn a lot more powder in exchange for a modest increase in performance.