Private browsing mode and deleting your
local history doesn't stop Google from keeping its own log of your searches (and any other Google services (gmail, youtube), and pages with google-analytics, and any page with AdSense,
etc)
Google clearly uses these logs to "customize" your search results, even when you check the placebo button to "disable" your history in the google account options.
Not having an account (or deleting local history) doesn't matter much, as they reconstruct a PK from all the entropy your browser leaks. (see their recent "we already ID your browser" reCAPTCHA update that no longer shows a scrambled image)