If you get it past rank 3, the game will only register 3 hits. I have it at rank 5, and I only ever see 3 damage numbers show up. The enemy it is hitting took 4 bats to kill before the ranking up and will not die when hit with 5 bats. When hit with my weapon, will die to just one hit from it, which is about the same damage the bats do.
If I would hazard a guess, the game has a set maximum number of hits that can happen in a frame and the bats are so clustered that they can hit all at once. I would either change them to cast slower or be more spread out.