There are two different levels of sales for books: the numbers that sell from publisher to bookshop; and the numbers which sell on from bookshop to reader.
That first number, which is usually referred to as the sell-in figure, is always the higher of the two, because a few books will always get lost, stolen or damaged; and so long as returns are allowed, many of those books will eventually find their way back to the publisher: sadly, returns rates of 30 or 40% are not unusual.
The number of books sold from bookshops to readers is usually called the sell-through, or the sell-on, and it’s a very different thing. Readers tend not to return books unless the pages fall out as they turn them, or a segment of the book is bound the wrong way around. As the second figure is more fixed than the first it is a far more reliable indicator of a book’s real sales, even though it is usually much lower.
So while publishers will often use the sell-in figure to trumpet a book’s success, writers should not rely on it when estimating what their royalties are going to be like, as those will usually be calculated from sales figures even lower than the sell-through, thanks to the joy of the reserve against returns. Which deserves a whole series of posts of its own!