Do you have a source for this?
I'd be willing to bet that the ONLY reason why 15% of their daily queries "haven't been seen before" is because they add un-needed complexity like fingerprinting. You're making it seem like they've never seen a query for "cute animals" before when obviously they have. They choose to do a lot of extra leg work because of who you are.
So your claim that 15% of their queries have "never been seen before" is probably inaccurate. I'd be willing to bet that "15% of their queries are unique because of the user, location, or other external factor separate from the query itself."
They've seen your query before. They've just never seen you make this query from this device on this side of town before.