The thing about most of these endgame bosses isn't their defense, but they have a built in chance to dodge all abilities, like amazon passives if you will. The amount varies, some bosses have 25% and some scale all the way up to 75%. I am the only person with this information, maybe I will find a way to get it where it is easy to read one day but it is actually a lot of work and isn't high priority. Regardless, even if your attack rating says 95% chance to hit it isn't taking into account the chance to dodge.