I think you're misinterpreting what it means by "given chance". When they say this, they don't mean it's a given, i.e. guaranteed to happen, they mean that the chance that it will happen is a fixed number (someone did a breakdown on this somewhere, given chance is like 30% and high chance is 45% or something like that, don't quote me on it). Of course that number isn't really fixed, it goes up based on your own stats versus the opponent you are facing, the higher your combo/control compared to them, the higher the chance. One thing I have noticed, is the in PvE, crits are actually a lot rarer than in PvP. On my slot 1, I crit my human opponents, about 60-70% of the time. In PvE, that number is more like 30%. So basing your observations off strictly PvE fights may not be the most accurate test sample.