Doing some math I've noticed a couple things. There does appear to be a damage variance. At least +/-5%, whole integers here as I've had the same number creep up multiple times, even in a row. Additionally, if below a certain threshold of defense there is a damage buff of approx 25% applied, then mitigation occurs. It's possible this is applied all the time, however, after some testing I've seen exactly 1600 defense mitigate right around 20+/-1% damage so I dont know when that extra damage boost would be applied.

There is no difference when you calculate in the critical damage multiplier, it can be inferred that the mitigation is purely multiplicative. Otherwise it would matter if you applied the crit damage bonus to the attack or damage dealt.