This doesn't mean much. Cataclysm was the first WoW expansion you could buy digitally, so retail was a much bigger deal. Now though, almost everyone is buying directly from Blizzard, digitally.
Stand in some bullshit line with smelly people, or push the button that gives you the game instantly? EZ choice. Unfortunately this means there is no reliable way to get sales numbers that are even remotely accurate for this title.
Every time you preorder a game, you become part of the problem. Don't be part of the problem.