As you can see, it's all a bunch of voodoo. The only people who have a clue as to how many books have been sold is WoTC, and even they don't know for sure.
Agreed, which is why they only said "hundreds of thousands sold" instead of "386,478 sold". They may not have given a exact number, but they did give a solid "ball park" range, which is certainly below 1 million, total. When you take into account there are at elast 3 books in the "core" then you can get even closer by dividing 999,999 by 3, and know that the books sales are no higher than 333,333 for each of them.
You can then estimate even better numbers by using excepted ratios of 6 players to every DM, or 3 to 1, and from there you can come up with very solid estimates of how many DMGs, PH's, and MM's have sold.
No matter which numbers come up, they certainly didn't serve a customer base of 6 Million fans. At most 2 Million, more than likely there are less than 1 million players of 4E. Most likely right around 500,000.
So people can call my estimates assumptions or notions, but WOTC is still the one who tried to push the perception that they had 6 million D&D fans when they clearly know they do not. Certainly not for their 4th edition, but I guess people defending WOTC shows that their marketed perceptions are bought into, no matter how clearly their newly revealed numbers prove otherwise.