YouTube calls that "validating views" sometimes bots will view a video, much like you'll sometimes bots commenting advertisements in your comments. It's basically quality control.
Think about it this way, If you had 110 views, but 30 of those views only watched for 10 seconds, that would leave a bad overall impression on the 110 views overall. But if 81 of those views had consistently good watch time, that leaves a much better overall impression on your video.
Other times there are inconsistencies in reporting. Views that seem to disappear, will re-appear later because of a reporting glitch. It happens more than you'd realise.
It's always a bit disheartening to see views wiped, but think of it this way, you'll get more quality views in time anyway, and in that time you spend being sad about lack of views, you could be spending that time into crafting your next amazing video.