The math shows that the gear alone provides them with a bigger advantage than us. Gear ilvl works like this -
Say you have an item with 1K stamina. You get the same item, 1 level higher. It'll now have 1010 stamina. You level it up once more, so it's now level 2. It'll get another 1% increase, 1010+10.1, or 1020.1.
It's a minor thing, but the advantage falls to the asians, as the bosses are "only" getting 8% more dmg/hp, while their gear gets exponentially stronger.
Add things like haste-softcaps (specific points where a DoT will tick once more for casters) that are more easily aviable due to bigger amount of stats on higher level gear (giving them crazy dps boosts when triggering a new tick), the fact that they can farm both 10 and 25 man every week for extra gear (tier tokens and trinkets for one, giving them even bigger dps boosts), and you've got something that can't even be remotely compared .