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Ok, I got some ideas now (but somebody must know for sure):
1. Google predicts step #2 the user will take after entering the site in question (using toolbar data).
I.e. you search for "netflix" (google then knows from toolbar data that a certain percentage continues to "Browse Selection" - and then shows that link in order to improve the user experience.
2. Google recognizes sub sites that get high rankings on the same keywords shown in
3. Some of both but also something else...