There have been some users reporting they get ads despite being Premium users, especially in regards to Youtube’s efforts to bypass adblockers (even if they weren’t using one). I always assumed that’s because their measures were misidentifying the lack of ads as using an ad blocker, even if that lack was due to using premium.

Just wanted to give an actual explanation. I’m not qualified to actually confirm or deny whether those user reports were factual or made up. But people usually consider them factual because of herd mentality.