Methodology(from their site):
* Population - amount of level 90 characters that meet any of the two conditions:
- The character is from a guild and the guild killed something in Tier 13 content
- The character killed something in Tier 13 content
What this means is the 1/0.55 ratio comes from the number of lvl 90 toons who are presently in guilds who killed stuff in tier13.
Why is this the most accurate (yet imperfect) representation of our current server balance?
a)The site reset their recordings yesterday for MoP raiding and most people do not yet have 2 lvls 90
b)It is the only site (for now) to not count inactive accounts by only counting lvls 90. This way it automatically eliminates the inflated numbers of other sites who still keep track of inactive lvls 85 (like the hundreds of Ruinous alliance toons for exemple).
c)Some may argue it only keeps track of PvErs but due to the "character killed something in Tier 13 content" condition and the fact that most pvpers got PvE trinkets or weapons last expansion, those numbers also include the pvpers who got to lvl 90.
Keep in mind lots of people are still leveling their first toon to 90 and that the ratio may change accordingly, but I do think it gives us a good insight of our current server balance/activity, at least at max lvl.