Well, some of the documents found were from when Biden was VP. So he did have them when he was not in office.

That said, it looks like an oversite on Biden's part, they informed the required parties and returned them immediately after being discovered. trump deliberately took, and refused to acknowledge he had them, refused to return the classified documents, and the government had to go get them after he was in office. And they keep finding more and more. A pretty big difference.