To be honest, I have lost a little faith in Google's system over the last 2-3 years!!
From hearing a lot of feedback from other webmasters, Google's structure of website indexing is not as clear and obvious as it once was. I'm not saying that all their SEO systems are questionable, but personally, I prefer BING's methods and general search results, as they appear to make more sense to me.
When the keywords you stress in a Google AdWords closely match the content on the subsequent landing page , Google will view your site as providing quality search result and assign your website a higher Quality Score
In saying the above, we have worked hard on general SEO over the last number of years, where everything from domain names, page indexing and content have all been tweaked along the way. In that regard, Google has pretty much rewarded us on common search criteria for our website.