This has to do with how copyright laws changed over time, for example there used to be a requirement in the US for works to have a copyright notice, or for copyright to be renewed, so some things that didn’t meet those requirements became public domain earlier than they could have if the copyright holder had cared about all the formalities.
This has to do with how copyright laws changed over time, for example there used to be a requirement in the US for works to have a copyright notice, or for copyright to be renewed, so some things that didn’t meet those requirements became public domain earlier than they could have if the copyright holder had cared about all the formalities.