The portion of those 2.5 million earmarked for Europe, for example, just got there today, and it's only been in stores for around two days in North America. All we know based on this is that Namco expects to sell a ton of copies, not that it is selling a ton. Although we're sure it will.
Maybe that's the idea -- to build excitement about Tekken 6 by showing how confident the publisher is. To print up 2.5 million of the things (some of which come in totally huge boxes), Namco must really believe it's on to something!