maybe 10 grains lighter, but not 10 GPI lighter.

XI32, a lot has to do with your (and his) FOC. If you have a higher FOC, your arrow will have a less flat trajectory because it is more front heavy. It may also be a tuning issue. If he is shooting a well-tuned bow and yours needs some work, a difference like this could happen easily, even with the differences in draw length.

Another reason could be vane choices. If you shoot a four inch vane with a strong helical and he shoots straight blazers or something like that, his arrow will naturally travel flatter because there is less drag down range. The more helical you have on your fletchings the better (and faster) you will stabalize your arrow, but it was also cause more drag and cause a greater change in trajectory.

Hopefully this helps some. If you want to shoot flatter, try tuning your bow as well as you are able (see sticky's at the top of this forum), use a smaller fletch that is straight fletched, and get your FOC closer to 5% than to 15%. The lower your FOC, the more balanced your arrow and the more it will naturally want to fly flat. But, it also will be harder to stabalize and tune if you start shooting fixed blade broadheads.

hope this helps