They call it “dark traffic” - ads that are not seen by tech-savvy users who have excellent ad blockers.
Not surprised that its growing. The web is unusable without an ad blocker and its only getting worse, and will continue to get worse every month.
The secondary DNS isn’t for redundancy; machines will split requests across the two for load balancing. If you only have one running, you’ll end up with ads slipping through as the device still uses the default secondary DNS.