Hopefully I'll be forgiven for breaking it down - basically, the problem is that we make assumptions about WoW and its players based on the people we know and play with, assumptions that are not valid. To quote Taufmonster:
The people you see when you log into WoW, no matter which server you play on, do not comprise a representative sample of the people who play WoW.
That sentence might seem a bit surprising but I can assure you that it's 100% true. The primary culprit here is engagement bias, which is something you have to consider when you're analyzing a game-as-a-service, like WoW.
The analysis that follows is pretty fascinating, and I'll leave it for you to go read it yourself: it's well worth it even from the perspective of someone who doesn't do a lot of number crunching like myself. I especially liked the point about how you collect your information. That by sampling the readers of a site that's dedicated to World of Warcraft like MMO-Champion in his case, he's already selecting for players who are more engaged with the game. Since this is the first part of a series of posts, it's definitely worth paying attention to Taufmonster's Log for more installments as they come out.