Do you mind me asking which boss fight has 3 targets constantly close enough for chainlightning over minutes? Maybe 2 targets for Dark Shamans.
But yeah, the result is not surprising. It is well known that crit and mastery are good for cleaving. But only mastery is also decent for single target. That is why crit usually isn't taken into consideration.
Oh, and of course: If your results show something different than for many other people, it doesn't mean something is wrong, but rather that your personal setup simply requires a different, individual approach because of certain circumstances (Meta Gem yes/no, Legendary Cloak yes/no, Rppm trinkets yes/no, different talents).
The only reason elemental doesn't like crit against a single-target is because a large percentage of its single target damage is auto-crit. The more you're allowed to spam chain lightning, the less of your damage comes from lava burst, the more you benefit from crit. Elemental has 250% criticals (most specs only have 200%), so this effect is very pronounced whenever sustained cleaving is introduced.
The thing is, unless you're having troubles meeting an aoe/cleave DPS check (e.g. p1 galakras or p1 garrosh, neither of which are terribly strict DPS checks), there's little reason to go out of your way to stack crit, since you won't be doing your single target DPS any favors.