My message went from seen to delivered on fb messenger, what does that mean?

I would think "delivered" would mean it arrived in the intended's inbox. "Seen" would mean the user you sent it to actually opened it. I've never heard of the status changing like that.

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Your question belongs in Computers & Internet>Internet>Facebook as it's currently in Yahoo Products>Yahoo Messenger (which hasn't existed for several years now) and not applicable to this subcategory (and thus decreases your chances of getting many useful answers).

Moving your question can be done by clicking Edit (pencil icon for mobile)>Edit category just beneath your question, then selecting the category that's appropriate for the topic.