Unless you're at a crazy height shooting something very close to your tree, the difference between slant distance and horizontal distance is almost meaningless (less than 2 yards in most cases). It's simple trigonometry...run the numbers and you'll see there's very little distance if your stand is 16 feet up, shooting at 15-30 yards.
Many people forget or don't know this and just lower their bow arm to aim down. This changes the relationship between the head and shoulders and the view through the peep sight. The result is usually a high hit.
This answer. So much this answer.
This is the reason the vast majority of people hit high from a tree stand, and not anything to do with the differences in range.