I would see it as the energy driving the pellet is tailing off over it's flight, ie, it's starting at 10 lb/ft when it leaves the muzzle but by the time it reaches the end of it's flight, it's producing far less than that! Therefore it's not consistent for a number of factors, ie, drag/resistance, gravity, etc.
If I misunderstood, I apologise, my mind isn't what it wasn't!

ASM