It's just a psychological thing. It makes the "bottom" of the side appear heavier than the top. You'll often see this same approach used by picture framers. If a picture is matted, the mat is often left wider at the bottom than the top. If all sides of the matt are equal, it sometimes appears as if the bottom is actually thinner than the rest. It's all for visual balance, so it doesn't trouble our brains. Just because a proportion is traditional, though, doesn't mean it's the only way to do something. Many, if not most classical guitars use an end graft with no flair, yet my brain remains calm.
