Hello,
that is basic physics and applies to about everything in the universe about 30 seconds after the big bang which moves slow compard to the speed of light.
Power = Force * Velocity.
Unfortunately as European I am more used to Si where 1 Watt = 1 Newton * 1 m/s
With 1hp = 745W and 1lb = 4,448N and 1kn = 0,51444 m/s
745/(4,448 * 0,51444) = 325,5
Here you have your fudge factor for this weired US system.