Contracted form of the Old Norse name RAÐÚLFR (or its Norman form Radulf). It was apparently introduced into England by Scandinavian settlers before the Norman conquest, though afterward it was bolstered by Norman influence. In the Middle Ages it was usually spelled Ralf, but by the 17th century it was most commonly Rafe, reflecting the normal pronunciation. The Ralph spelling appeared in the 18th century."
The pronunciation pre-dates the current spelling, it would seem.