Let's not forget that on level ground gravity immediatly begins to affect the arrow, so you are aiming the arrow up in an arc to make sure your arrow is on target when it reaches it's goal. From an elevated platform gravity has a lesser effect on the arrow. This will throw a slightly straiter trajectory to your shot,not much though. But combine the distance thing, the gravity thing, throw in a little bad form and viola' you've just shot high by several inches (the form thing being the worst).

My advice, climb up that tree with a quiver full of arrows and have at it. Remember to bend at the waist.