Simple: asian realms have 8 ilvl higher loot in 25mans (and bosses having more hp/do more dmg to compensate), aswell as them having seperate lockouts for 10man and 25 (atleased i heard ppl talking about this 2nd part) thereby gaining gear faster then the west. To put things in perspective, the 10% boss hp and bossdmg done gets outscaled by the ilvl bonus, hence they have a harder time for their 1st kills, but anything beyond that makes the fights easyer for them then it is for american/european guilds.
Ghostcrawler wrote somewhere that 1 lvl is about 2% dps increase (from that item) so asians have 16% more dps from gear, while being required to push 10% more.
Hence why there are seperate rankings for asia and for eu/us, as it is a unfair race.