I am not suggesting YouTube know who my daughters godparents are or who my previous partners were. But, without registering should they know my musical interests this well?
If you search using Google while signed in with a GMail or GApps account, YouTube may pick up on interests from your search history. Cookies!! Although more likely is your previous viewing history which will be cookie based. Try watching in an incognito window and see if the same recs are made.
It looks like it is my browsing history as you suspected. I am guessing there is a cookie identifying me as opposed to my explicit history. Anonymous browsing is offering me videos that I suspect more people to watch and few if any that reflect my personal preferences.
It’s obvious that they’re not making their Activity History very obvious when there are geeks who have it enabled. I disabled it not for privacy related reasons, but because I wanted to see what is recommended based on what is related to the music in question, not based on what I listened to 2 years ago as that’s what it did before. It does still use your liked videos into consideration.
Please respect our code of conduct which is simple: don't be a dick.