This is likely how chain heal and similar smart mult-heals work. When you start casting the heal it figures out that chain then and at the end of the cast it does it as planned if it can.

Likely there is a similar effect when you divine storm at the same exact time, the heals are targeting the lowest guy multiple times for each paladin and then ignoring the other needy characters. Even if the first heal overheals. Just because of the latency involved.