If I understand correctly, MS saying they sold 6 million units is correct.
But you fail to understand that MS sells units to stores, not customers.
When MS says "sold" they mean that they put 6 million units in shops, the shops bought them from MS, so, for all MS cares, they are "sold".
Either way, as I described above, even if the total was 4.5 million, the 360 would be WELL above 7.9 million. That figure is just completely out of whack, and way out of the realm of reason. If the 360 is found to be below 9 million by Dec 31st, I'll eat my socks.
I'd say the same thing if somebody called "360_gamer" tried to make a site and say the 360 sold 12 million units and cited several unknown sources. I'd tell them they are insane.