Badmoon

I believe there is a to-hit roll for every mob in range. Just using a normal curve (bell curve), the odds of missing some are guranteed. I'll notice the misses more when there are only a few mobs.

I have also tossed a Fireball into a group of 8 or more hoping it would miss most, and it hit the vast majority of them. All I can suggest is slot accuracy until your at a point where you like. Dont expect it to hit em all.