Apparently it ranks based on date cleared compared to the 'world first'. It seems to been at least an accurate indicator of the top three guilds relative to this post. Correct me if I'm wrong, but only one person in the FC would have to have the achievement and have it set to public for it to register the FC and then check the dates compared to 'world first'.
Additionally, this post is inaccurate as well after the top three, heaps of guilds haven't registered their interest. Seems to be decent for cross-referencing at bare minimum. Still surprised no-one linked it prior (or I'm blind). But thanks for the heads up, it's probably for the best its accuracy was questioned so people didn't cling to it. I didn't ferret like I should've, I'm a bad.
http://www.xivarmory.com/faq
That's my understanding of it anyway.
Also, compare them ilvls to see who has a leg up on the race to Twintania server first man, that is where the real fun is!
P.S it's not me, tank shit never drops.