Just because you can't explain it, doesn't mean it's a miracle. Religion is a controversial business in the Discworld. Everyone has their own opinion, and indeed their own gods, who come in all shapes and sizes. In such a competitive environment, there is a pressing need to make one's presence felt. And it's certainly not remotely helpful to be reduced to be appearing in the form of a tortoise, a manifestation far below god-like status in anyone's book. In such instances, you need an acolyte, and fast. Preferably one who won't ask too many questions...