@Arc Both the book and the scrap of paper are written in Old Icelandic but the runic scripts used are different. The book contains the letters ð, þ, ö and æ that don't exist in the Latin alphabet. Writers of the 12th century often used a mix of runes (to transcribe Old Norse sounds that don't exist in Latin) and latin alphabet in their books. If you look up manuscripts of the "Codex Regius", the "Kringla leaf" (the last surviving page of the Heimskringla) or the "Prose Edda", they all look like the book on page 23. There was no standardized alphabet at that time so there's a bunch of variations in how runes are used.
Now, the scrap of paper found in the book and its transcription into Latin alphabet are actually copied from the illustrations of Jules Verne's book. That one's a bit different because the runes spell out a Latin message and they're runes from much later in time that someone knowledgeable like Axel's uncle should recognize immediately. I think that's an issue that's due to the manga because the book it's based on only had an illustration for the scrap of paper, not the book that contained it.
TL;DR: Not an oversight! Just different sets of runes used.
Hope that helps! It's a really interesting but complicated topic...