Have a look at this list of best-selling books. In particular, look at the Harry Potter books. There is one Harry Potter book in the list of best-selling books. Harry Potter and the Deathly Hallows, which has sold approximately 44 million copies. Harry Potter is also listed as the best-selling series of all time, with 400 million total copies sold.
In order to see why there is a problem with this, you just need to know that there are 7 Harry Potter books, and understand the Pigeonhole Principle.
There is a (sort of) explanation of the problem in the 'talk' page of the wikipedia article, but the inconsistency remains on the initial page. This actually highlights a fairly major problem with wikipedia's epistemology. The inconsistency has to remain because there's no 'credible source' for the figures for the other individual books. So even though everyone knows that the article is wrong, it can't be corrected because this would constitute 'original research'. Hmm... just how credible does a 'credible source' have to be? Will a mathematical proof do?