Area of Effect vs. Mirror Image

[Ironhawk]

In a game I played a while back, I was Tien'Chi and making great use of the spell Acid Bolt, which has an Area of Effect of 1, iirc. I was then attacked by a whole mess of Vans who I thought would be shredded by acid bolts as they rushed my mages. So, I watched with glee as my acid bolts arced over the battlefield and slammed into the Van formations, only to be shocked to see no deaths. "Well they are just injured then" I thought to myself and went to check the units in question. But I checked them and found that there was no damage or injuries of any kind!

I had thought it was the case that ball spells (anything with and area of effect) hit all units in the same square that they touch down in? Is that ability negated by the natural Mirror Image of the Vans? Or did I just not understand it correctly?