Google knows everything. Including which links people click when they've searched for something.
When you click a link on a Google search results page, it's not actually the address that's being displayed. For example, I just did a search for "wolverine" and here's the address that's displayed for the second result. (The first is a paid ad.)
When you click that link it tells Google that you went there in response to your search. I imagine the main reason is to help them rate followed links so they place higher in the search results later. But with a little back-office wizardry Google can probably also tell if more people were interested in a spandex-clad hero or a fur-clad buzzsaw.