I was thinking of posting this as a bug but instead I'm putting it here in case that it is intended. If someone can confirm that it isn't intended then I will post it where it belongs.
Using Arcane Missiles on a lower level enemy will make you cast a lower rank depending on the level of the enemy. For example, clicking on Arcane Missiles rank 13 while you target a level 8 enemy will make you cast Arcane Missiles rank 2 instead.