[QUESTION]Fireball have a hidden target limit?

I was in LEEC-The Demon Stink and lucked into a Fireball on initial draw. There were six Giant Cockroaches that were highlighted yellow. When it was actually cast, only four were hit. No armor activated, two different groups were involved, nothing in logs other than four took damage and two didn't.

So, bug or hidden limit of Fireball? Or did I miss something else?

 

Did the target square have line of sight to all six enemies?

 

I went back to the map to check... and looking at it, I'd have say it didn't have LOS to the two that got away. Having said that, I'm positive the square lit up yellow when I was placing the spell. Maybe I had a shaky mouse hand...