The transitional time in January at the end of the Presidency and Vice Presidency is a mess. You moving documents, personal items, etc to your personal home from the white house or VP house. Its probably a lot of documents being moved and they don't look through every single one to see if its classified or not.
Seems like there needs to be a better system in place. It wouldn't surprise me if several former Presidents or VP's had classified documents after they left office.