Crit mechanics question

dvstansb14_ESO

I don't normally run crit builds (main is a tank), but I've been leveling a NB alt and have noticed that I rarely get what seem to be "super-critical" hits against PvE mobs (and I think other players too). In some cases, the target is stunned for a few seconds, in others (when using DW poison enchants), the target is insta-killed (from 90% health to dead in one hit). My crit rating is about 55%, crit dmg is default, and I don't have any gear bonuses that would cause either of these effects. I basically crit every other hit, so I don't know what causes these other rare effects. Anyone know what's going on here?