This pretty much captures it. Google knows a lot about you, and it knows a lot about the people you hang out with and the kinds of things you read and the kinds of things you don't read. In knowing all of that stuff they can ascertain that you are part of a community (well a cohort at least) with certain preferences and proclivities, and as soon as a new user identifies as also part of that cohort they can tweak the user experience to "improve" it.

And don't get me wrong, it does improve. I'm glad that I'm not bombarded with ads for TJMax on Google properties but think Digikey could back off a bit.

From a feedback perspective though it gets weird. And you can see that when you search without any identifying information enabled. I found the whole 'Don't Bubble Me' campaign quite creative.

The challenge for search engines though is that being the reference librarian's favorite tool for objectiveness doesn't woo the mass market. And search is a game of numbers if nothing else.