From what I can tell, Google cares a lot more about recency.
When I switch over to a new framework or language, search results are pretty bad for the first week, horrible actually as Google thinks I am still using /other language/. I have to keep appending the language / framework name to my queries.
After a week or so? The results are pure magic. I can search for something sort of describing what I want and Google returns the correct answer. If I search for 'array length' Google is going to tell me how to find the length of an array in whatever language I am currently immersed in!
As much as I try to use Duck Duck Go, Google is just too magic.
But I don't think it is because they have my complete search history.
Also people forget that the creepy stuff Google does is super useful.
For example, whatever framework I am using, Google will start pushing news updates to my Google Now (or whatever it is called on my phone) about new releases to that framework. I get a constant stream of learning resources, valuable blog posts, and best practices delivered to me every morning!
It really is impressive.