This question has many possible answers, because you may get unknown and not watched videos in YouTube video watch history in different situations.
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Possible situation where un-watched videos saved in YouTube history
1. Your friend, family member or other trustworthy person can access your phone or other device and watch such videos on YouTube with your login. In that case, videos come in the history list, which you have not watched.
2. You are watching a video and in between get busy with other tasks and the next video starts playing automatically without your knowledge.
3. In the YouTube app, when you stop scrolling in the YouTube homepage video feed, video plays automatically with the inline player feature. In that case, if any video plays for 10-11 seconds, then it will be saved to your watch history.
4. Next, you may access YouTube with login on a public device and forget to logout.
5. Someone knows your Google a/c credentials and access YouTube with your login and watches such videos.
How to know that how un-watched videos saved in YouTube history
There is no sure shot way to find how un-watched videos are saved in YouTube history. But, you can get hints based on un-watched videos.
For example,
If there are few unknown videos in your watch history related to your choice and some are few second watched, then such videos may be there because of auto playing or playback in feeds feature.
Next, you can ask your friends or family members to confirm, if they watched such videos on your device or with your login.
At last,
If you find many unknown videos in your watch history and are not able to find clear reasons, then it is good practice to reset your account password, logout from all devices using Google account Manage devices settings and login with a new password.