Google for the "long tail" and you'll get to understand that the top 13 is not where you want to target. The top 100 search terms account for just 2% of the total traffic. The remaining 98% of traffic are for all the mirriad of other search terms. Clearly there is more money in the 98% of traffic for unpopular keywords than there is for the 2% of traffic for the 100 most popular keywords.