Simple answer is because nothing like 10.4 million units have been sold, as many have stated there are a huge amount of units sitting in warehouses and in shops - same with PSP.
VG Charts tracks sales not shipped numbers generally, as in some cases such as this there can be a large discrepancy between the two...
It is well known that MS reports "sold" as "sold to retailer", whereas Sony reports "shipped" as a unit that simply has left the assembly line.
The difference is that the MS total does not inlcude those units just sitting in a warehouse unsold to retailers, whereas Sony's could. Their "sold" number is units that are in the consumer's hand or sitting on the store shelf.
Here's how Robbie Bach phrased it:
"Robbie Bach on how they count things:
Q: Itâs 10 million shipped, right?
A: Yeah. In our case, itâs reasonably close to sold through. To make sure weâre clear, Sony does shipped from factory. We donât. Our shipped means it has left a distribution warehouse in Memphis to a retailer. There is a big lag of six week to eight-week lag between what we called shipped and what Sony calls shipped. Thatâs the way we do the accounting."
Where is this "well known" information about Sony's sold/shipped/sitting in warehouse consoles coming from? I'm not inclined to just adopt anything Microsoft says about it's competitor's accounting practices. Sony specifically states "shipped" when it talks about it's sales numbers, just like Microsoft.