It's a surprisingly common experience for music students to excitedly tell everyone they know about a new piece of music they've been composing, saying it is probably the best thing they've ever written, and then a friend or teacher has to say, "I don't know how to break it to you, but you've 'composed' the Xth movement of Beethoven's Yth symphony."
And sometimes they will say, "I have? I don't think I've ever heard Beethoven's Yth symphony." But of course they have, just without realizing it. It was in the background of some movie they watched or something like that.
Unlike humans, I don't think AIs have any belief about whether their work is original or not, but it's the same type of error. And with similar legal consequences: people have been sued for stealing a melody (presumably not always consciously). The difference with AIs is they can produce much more output than humans, and it's muddier what is actually doing the creating (AI authors? users?).