By default, Google keeps track of user browsing histories. Many game players are also Google users, so they can follow your game-playing habits from the time you searched "XCOM release date" to "XCOM walkthrough." Some interesting tidbits:
-27% of tip-seekers browse from a mobile device.
-84% of game sales four months after launch can be predicted by Adwords clicks.
-Pre-launch search interest is in publisher-released material.