The have to put in some out of context ads sometimes, just so they can train their system.
For example they may show you an ads about hairdryers after you searched for computer keyboards. There is absolutely no relationship between the two, but Google doesn't know it yet, so they try. After all, there may be a correlation. In the likely event where there isn't, Google will simply stop showing hairdryer ads and show you computer mice instead (a known correlation) or running shoes (another experiment).
For example they may show you an ads about hairdryers after you searched for computer keyboards. There is absolutely no relationship between the two, but Google doesn't know it yet, so they try. After all, there may be a correlation. In the likely event where there isn't, Google will simply stop showing hairdryer ads and show you computer mice instead (a known correlation) or running shoes (another experiment).