I will preface this post by stating that this is all just conjecture and that I have zero hard data (just experience) to back it up.
In my experience, it seems like AoE attacks don't necessarily follow expected crit numbers. More often than I'd expect, I'll hit an AoE attack (for example Raging Blades) that throws out a huge number of individual attacks and either all or almost all of them will crit, or none will crit. On a pull with 6 mobs, this is 18 attacks. I'm no math wizard but it seems like, with a 20-something percent crit chance and 9 percent-ish devastate chance, it would be extremely unlikely that, out of 18 attacks, none of them would crit or that all or nearly all of them would crit.
Any ideas? I'm thinking there's different rolls going on, where somehow the crit is at least partially determined by a roll against my target rather than against actual hit mobs. Maybe I should test what happens using AoE attacks when I don't have anything targeted?