Does anyone know why they chose "ronna" instead of "renna"? It should be r + ennea (Greek for nine), so where does the "o" come from? Every prefix up until now

Does anyone know why they chose "ronna" instead of "renna"? It should be r + ennea (Greek for nine), so where does the "o" come from? Every prefix up until now has been consistent about the first vowel mirroring the Greek word: tetra -> tera penta -> peta hex -> exa hepta -> zetta okto -> yotta