Although labels can easily lie, they are the only entity that knows how much an album has sold since they're the ones having the album made and shipped out. The fact that people think Chart News and Media Traffic, both of which are ran by people like you me, know more about sales than actual labels is just

.
And the whole "those are shipments, not sales" thing is wrong. Shipments are sales. Do you think labels ship out albums for free?

There has to be demand and stores have to order these albums in order for them to be shipped out. They are bought by the store, making them official sales, which is why labels count them.