hornbow

This is correct. In more mathmatical terms, the flight of the arrow can be considered a VECTOR.

Draw a diagonal arrow on a peice of paper. There are both horizontal and vertical vector components of this diagonal vector. The vertical component is equal to the magnitude of the diagonal vector multiplied by the sin of the angle it makes with a vertical line (the horizontal component is magnitude * cosin of the angle it makes with a horizontal line). This vertical vector component in the case of arrow flight is equal to the force of gravity. So, the steeper the angle, the less gravity will effect the arrow's flight and effective range.

Think about the extreme case of being infinitely high up and firing an arrow straight down. Gravity would not effect the arrow's flight path or range at all! You could theoretically shoot at and hit a target 100 miles straight down.

In practical terms, this means the higher you climb, the more your effective range is extended. With my compound bow, I have found I gain about 1 yard of range for every 5 feet I climb. At 15 feet up, my 25 yrd pin hits dead on at 28 yards. This may well differ for different bows however so I wouldn't use that as a general rule of thumb.