Weird Damage

I was fighting against the Phoenix and for some reason every few hits Ashe would do about 700 damage. I have her equipped with the Ragnarok sword. She normally does between 3000 and 4000 per hit. I know that it couldn’t be the fact that either he was strong against her attacks because all of the other attacks she was landing were doing normal amounts of damage, or the fact that the sword does fluctuating damage, because it doesn’t. I also know that it shouldn’t be Telekinesis affecting it because both Vaan and Basch had the Save The Queen swords on and nothing effected their attacks.

On a side note, while getting to Phoenix I kept hitting these "swarm" traps. They seemed to have no effect on me, and I was wondering if anyone knew what they actually did.

Easiest most plausible awnser to me, seems, critical hits.

So a critical hit can do less damage?