I'll be an adult here and propose that Blizzard doesn't actually owe us their weekly or monthly unit sales sales numbers. It's a fact that great and record-setting sales numbers are worth a press release and Blizzard gets those often enough that it can make it seem like something they always do since they have a history of setting sales records on a regular basis and then talking about it. Press releases about sales are just that: press releases.
Where the logic goes awry is when someone implies that because they didn't set records, it's somehow a failure. For all we know, Blizzard's sales projections may have been higher than what actually happened or MoP may have surpassed them. In any case, Blizzard is highly unlikely to provide their internal forecasts about what they expect. Blizzard does owe its stockholders some information and that is pretty much legally defined and on a quarterly basis is fine as far as it goes.
Threads that vaguely hint that Blizzard is somehow failing at something because they're not shouting out their success at 100 decibels miss the point. They're doing just fine.