The benedictine thing is a claim and tradition of the benedictines themselves, perhaps because they are also called "olivetans". The author of the prophesy himself never said it would be one of them. I have also heard it said that the original prophesy only listed 111 popes, and that 112 was added in an edition long after his death.
The more interesting factor is that there's only one or two popes left according to the prophecy, so I guess we'll know soon enough!!!