I have no idea, but I think it's just how the boss is timed.
The WoD-Legion gap is definitely wider than the MoP-WoD one. Being able to kill the last boss of the previous expansion so early into the next one, even in the lowest difficulty mode, is something new for Mages. I think that has to do with Legion gearing system and legendary items. Honestly, legendary Robe does 50% of the kill, because without it I had to equip sub-optimal Leech gear and sacrifice DPS for kiting. However, this is only because of the reset mechanic. If he wasn't doing that, he would have been killed much earlier.
Next step is Mythic Highmaul and Heroic BrF+HFC !