deploying multiple lightning spells rapidly

I saw someone rapidly deployed several lightning spells on my AD and killed it. I had always done this, but I did it slowly, because I was afraid the full damage wasn't registered.

So when I saw someone else do it to me, I decided that it was safe. So later on, I deployed 3 level 4 (390 damage) lightning spells against a level 5 AD (990 HP). And to my surprise it didn't kill it, and totally ruined a perfect revenge for me.

So is this a bug? Does it only work sometimes?

I was near the edge of the AD but definitely within the square. The person who did it to me actually did it on the edge touching other defenses and still got it to work.

Check to see if their lightning spell is a higher level than yours.

In my experience, it does make a difference. I leave about 1 second delay between lightnings if I ever do multiple. I usually don't fully wait for each one to finish (unless I'm trying to take out multiple defenses and I need to adjust my lightning spell placement based on how much HP their defensive units have left after each lightning) before starting the next, but rapidly spamming them without any delay seems to do less damage from what I have been able to see, both through my own practice and from watching replays of lightning spam attacks against me.