From what I've gleaned elsewhere, "ter-" implies a lack of active agent (or accidental action). If you forgot your phone, it is "tertinggal", because you did not do it on purpose; you did not act. So "ditulis" would be used if you refer to an agent who wrote this: it was written (ditulis) by my sister. But in this case, we're not talking about who wrote it, so it is "tertulis". Both kind of mean the same thing but the emphasis differs. Take this with a grain of salt though.