Put simply, the speed of the ground being shoved out of the projectile's way is equal to the speed of the projectile, and it cancels out.
To expand on this point.

If I drop a stone on sand from head height, say six foot, it displaces x amount of sand. If I throw it from the same height, it displaces more sand and leaves a larger crater. The stone will be lower in the sand, compared to when I merely dropped it. Right? This isn't logical to me right now.