I agree with Jollyarcher. Since you went to lighter arrows I'm assuming your diameter of your arrow changes which in turn would change the tune of your bow with the new arrows. It's likely the arrows are not coming out of the bow clean thusly giving you the high hit as you back off twenty yards. It has to be in your arrow flight because an arrow starts to drop immediately out of a bow similiar to a bullet...that's the law of gravity regardless of the upward trajectory...if your hitting higher at 25 than you are at 20 with the same pin, it's arrow flight...thus tuning.