EXACTLY!!!! Sometimes it ain't the bow - it's the broadhead. Even out of a perfectly tuned bow from a shooting machine, different broadheads will impact different spots on the target. Look at this test from a shooting machine with a well tuned bow. Notice how the point of impact varies just by switching broadheads. This is also true for mechanicals but to a lesser degree....
http://www.mucc.org/BroadheadTest.htm