One theory I have is that for certain categories which have high paying AdWords, Google will penalize results which have high CTRs on the regular search results so that people are more likely to utilize the AdWords.
Conspiracy theorists would say it drives searchers to AdWords where Google have an income stream. If they have their aim set on short term gain then this has some weight.
However, what would be the cost to Google if such tactics were revealed to the public? It would only take one disgruntled ex-employee to blow the whistle. The damage would be enormous - trust would be destroyed, searchers may switch to a more ethical search engine and (in the face of an IPO) the company's value could collapse in the eyes of potential investors.
Isn't it more likely that Google need to keep squeaky clean and are genuinely trying to deliver the best quality organic results?
Good organic results keep searchers loyal to Google. If you trust the results you use the engine more often - if the relevancy falls you use it less often.
So, relevancy of results increases search volume and increased search volume increases AdWords sales even if the %age of AdWords clicks per 100 searches stays the same.
Just my two penn'orth..