Because no one has the exact same set up and specs and the next person.

I have a 30" draw and pull 70 poundsand would get real close to IBO rating of 325ifI wanted to shoot a 350 grain arrow. However, I shoot a 500 grain arrow and get 275 fps.



IBO standard is 30" draw, 70 pound draw weight shooting a 350 grain arrow with nothing on the string.

AMO standard is 30" draw, 60 pound draw weight and shooting a 540 grain arrowwith nothing on the string.