I'm aware that moonflower vines are related to morning glories but I cannot find any literature explaining why both vines display 2 distinctly different leaves.

The majority of the leaves on both vines are perfectly heart shaped. However, the initial foliage of the moonflower has 3 spear-like points (kind of like the bottom half of a heart) I havent noticed a specific number of leaves until the change here. The first 2 leaves on a morning glory are more like 2 circles overlapping (or the top half of a heart).

What's the deal?!