Originally commented on the Exsteel thread in response to the wow subscriber number mentioned there, but this is a better place for this comment:

I personally believe that Blizzard fudges their numbers. Since they technically never allow account deletion (you can only terminate your current subscription but can reopen it and reactivate your account at any time) my belief is that Blizzard reports on the total number of accounts they have... not the total number of active accounts.

Anecdotal evidence such as lots of reports of "emptier" servers and the fact that the global wowarmory only contains 4+ million characters (at how many per account) seems to support this theory.