I have link it in the trinket discussion but I shouldn't have. We discuss about it with Lhiv and he found that he should have the wrong action list that still check for frostbolt's debuff for some casting. It's why dps is so low.
Actual simulation show a drop from 240k to 235k.
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and mage tend to be overestimated by a few % in 5.3 SimC. Don't now with 5.4.
So it's ok to compare between mage spec but not really with other class.
I don't no why. Maybe, it's because of L90, some problem between AT & BL, lag impact or something like this.