RE: Why??????
Leverdude is correct , when you move your rest slightly [1/32 quoted] your field tipped arrows should move 1/4 to 1/2 an inch at twenty yards , but your broad heads might move an inch or 2 in the same direction , if they don't react that way , you may have a weak spine issue or a torquing issue , allthough I usealy find thats shows itself in a right/left problem . Sounds like you just need to slightly lower your nocking point , or raise your rest .