[halkony]

Google can reasonably infer people that live together based on billing addresses, location, etc. If your roommates pay particular attention to certain media (a video, a song, etc), it's more likely to show up on your recommended media.

In this post, chances are that Google classed each user as being socially close and based its recommendations on that. It's uncanny, but with how much Google data has, eavesdropping is unnecessary for this use case.