As with anything Google it's all part of an algorithm that no one but Google knows. The one common factor is that this typically occurs only for the top ranked site and is usually a major brand/popular site. You can read more about it in Matt Cutt's blog .
stymiee
He's No Good To Me Dead
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I was under the impression that it was the top performing AdWords ad for the particular keyword. I could be wrong.
cscgal
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