Some books are more equal than others.
When publishers talk about a lead title, they're referring to a book (often, but certainly not always, by a début author) that they believe has the potential to blow out in terms of sales. These aren't the books by established hot shots or memoirs/self help books by celebrities; these are those front-of-store, gotta-read titles that seem to erupt out of nowhere. But nay! They do not appear ex nihilo, cats and kittens, but rather, they are built and designed: they receive six-figure marketing budgets, co-op dollars, aggressive publicity, additional sales materials. In short: the works.
Now, to be fair, it's not possible to engineer a bestselling title without the reading public's participation. A Big Six publisher could throw millions into marketing and co-op and not come anywhere close to breaking even if the consumer doesn't participate (i.e., purchase the book). That said, we all (as consumers, anyway) are much more easily manipulated than we would like to believe, and there is a direct correlation between the amount of money, time, and energy that a publisher puts into a book—a lead title—and that title's performance in the market.
Some lead titles flop terribly. Most, I think, break even, depending on the amount of money sunk into the endeavor. A few (far more than the average for books in general, but still not a huge number) become major bestsellers, and I'm inclined to believe they wouldn't have had they not had the big budgets and know-how of a large publishing operation. When the publisher pays for those big stacks of books at the front of Borders or Barnes & Noble, lines up interview after interview with major media, and advertises in magazines and locations you're likely to read and frequent, aren't you going to pick up that book and at least read the dust jacket? That's half the battle, friends: getting you to pick up the book. The publisher believes the content is enough to win the second half of the battle (getting you to bring the book to the register), but first they have to spend enough money to make it easy for you to find.
Not all major titles are/were lead titles (Harry Potter—the early books, anyway—being a good example), but classifying a book as such is a way of allocating funds and marking which books are believed to be wildly successful before they even ship out to stores. Is it fair? No. Is it practical in this industry? Absolutely. And by no means are the non-lead titles of the world doomed to failure—they are, as I said, "equal"—but they're not as equal as the titles that receive substantially larger investments of time, money, and effort, and the odds of their doing as well sales-wise is very slim indeed.