In the sales department, dear authors, we are beholden to two masters: publishers and buyers. The publishers are the folks in-house producing the books to be sold to book stores; as you've probably already guessed, the buyers are the ones at various retailers who are buying them. Publishers have one number in mind when creating a book to sell, and buyers have a (usually somewhat different) number in mind when they're buying.
When a book is acquired, a P&L ("profit and loss") statement is run, which (among other things) tells the publisher how many copies of the book they'll have to sell to break even given a certain advance to the author and a certain price point for the book.
Once this number is hashed out, a series of goals is created for each retailer (based on the P&L and selected comp titles). The major chains are assigned a goal, on-line retailers get one, independent booksellers, &c &c. The sales force then reviews these numbers and adjusts as necessary.
What this means, mes auteurs, is that the sales force is not ultimately charged with coming up with the highest number for a book, but with what they believe is the right number. Sometimes the publisher is disappointed because they think the sales rep's number is too low; sometimes the buyer will disagree with a rep because they believe the rep's number is too high. It's a delicate balancing act, and getting the two teams into the same ballpark is occasionally tricky.
What helps? Well, buyers are swayed by a lot of things, but chiefly they're concerned with publicity for the book in question, comp/previous title sales history, and the price. More publicity hits almost always help, especially confirmed appearances on talk shows or serious review attention (now shifting away from newspapers and toward bloggers). Good comps are, as I've said, important, as they also have a lot of influence on the final number.
As regards price: interestingly, a high price point will discourage a buyer from taking a large quantity, but a low price point won't necessarily encourage him or her (unless that low price point is communicated on the cover, say with a mass market edition that has a "Now $4.99!" starburst on it).
If you're ever unclear on your sales figures, you should feel free to ask your publisher (via your agent) about it (approaching the sales force directly is not a good idea). Knowing a little bit about the sales process for your first/previous book can be a big help to you when positioning and pitching your second/subsequent one.