[llamabeast]

I don't know, but I think it's something like this: The unit takes a check for each unit that it has to fly over to see if it gets distracted by it. If it does, then it attacks that unit instead. Obviously your pretender got distracted by the very first unit it tried to fly over.

This mechanic exists to stop "attack rear" being too powerful, and it also applies to units on the ground. For example flanking cavalry will often get distracted and move in to attack the enemy's front unit, but sometimes they will make it all the way to the back.