I have seen this with Music too, usually there is a small difference like Album (in the case of music) or Aspect ratio (in the case of movies) that can cause a difference.
There were news articles a few weeks ago showing you can't rely on the iTunes Store to keep your content you have purchased for downloading again later. You NEED to keep copies on your local system, with backups. I have seen this with several songs I own, as they are no longer available in the iTunes Store. In my opinion, you need at least 2 copies of every purchase, three would be better.
Thanks. That would explain the option to repurchase.
I currently have an 8TB drive dedicated solely to my iTunes library. It's around 5TB full.
I've wondered in the past what would happen if something I bought was withdrawn from sale so I download everything.
Backblaze then has the unfortunate task of keeping that lot backed up. I think with that amount of data, the 321 rule might be difficult.