Sony changes their number every time it fits for them. With PSP they started to announce number which was the number of units manufactured (this includes quantity in their own warehouses =shipped from the factories) to inflate numbers and make the console to look more popular, and now when they have to make PS3 look more popular, when they know that no one believes that PS3 have sold the number they have manufactured, they announce the number of sold to retailers (this does not include the quantity in Sonys own warehouses =shipped to retailers).
Of course the VGChartz numbers aren't accurate, since they are estimates, but atleast they are not numbers that are twisted to inflate the sales numbers. Sony doesn't necessarily have any better data about actual sales than what VGChartz have, since it's very unlikely that retailers report every sold unit to Sony when there are even importers between retail and Sony.