Well, part of it has to do with the arrow shaft size and weight. Front of center ( foc ) of the arrow is partially dependant on the weight of the broadhead. The foc helps to determine how the arrow will fly. Check out the link I am providing. This should help you to understand. The section for F.O.C. is around the middle of the page. But feel free to read the whole article. You might find some more valuable information.


http://www.huntersfriend.com/arrowhelp/arrow-selection-4.htm

This will explaain why you should choose a particular weight that will match the shaft and the set up of the arrow for optimal flight.

Good luck and good hunting

God bless