This might be naive but, in the age before spell check, I wonder if a lot of people consistently made spelling mistakes like this. I mean you wouldn’t know you were wrong unless you checked in the dictionary.
I think "paradice" is likely an intentional mistake on the Zodiac's part, though. His apparent level of education doesn't fit with him being unfamiliar with that word.
It's possible he did it intentionally but I don't think this cipher is proof he was highly educated.
In related news, Japanese teachers lament the fact that predictive text entry systems on their students' phones mean that they don't know how to write kanji anymore. They can recognize them, but don't have enough practice to write them consistently anymore.