Google announced a "quality change" to the AdWords ad delivery system. Danny Sullivan explained that the ads displayed on a search result page will differ in quantity based on the commercial nature of the keyword phrase.

So if a keyword is more educational (non-commercial) in nature, less ads will be displayed. If a keyword phrase is more commercial in nature, more ads (up to 11) will display on the search results page. How does Google change the number of ads? Well, if it is less commercial, it will tend to use less of the broad match criteria in displaying ads (which tends to bring up more ad inventory).

Find out more about the Adwords ads "quality change" at http://www.searchenginejournal.com/?p=3350.