General and Specific Keywords

If you wanted to buy the book Influence, would you type "books" into the search engines? Or "Influence, by Robert Cialdini"

If you wanted to buy a Maytag Washer, would you type "Washing Machine" into the engine? Or  "Maytag 27" Top-Load Washer with SuperSize"?

The former will render larger volumes of visitors, the latter will render less visitors, more sales. So you are likely to get closer to a 1 to 1 conversion.