There are a few variables I can think of right off the top of my head that can cause you to shoot the same bow to a different POI. Your face might be wider than his, or vice versa. You might anchor harder into your face than he does, or vice versa. You've already said you had to shoot short draw with the bow, so there's a potential biggie. Your hand and grip are more likely than not to be different.

Any little thing that is different in the way you anchor, hold the bow or even differences between your physiques can cause arrows to strike different POI's. There might be one thing that accounts for the whole 7" variance, but I'm more inclined to be of the opinion that there are several differences that accumulate 7" worth of variance.