jake s :
Im guessing around 75% of these numbers are probably from accounts that were perma banned.
Blizzard was running a promo early last year for a free D3 download to people who signed up for a whole year of WoW. I do not remember the date limit for that promo but it was probably just about now.
So the more likely theory is that the bulk of player losses were players who subscribed early in the promo. There will likely be some more losses from people who signed up near the end of that promo who have not been accounted for yet.