I'm afraid, this doesn't really answer the question. Plus: I'm afraid, nobody really can at the moment. The change from M to P has not been sufficiently explained (cf. Molly/Polly). Hanks & Hodges '92 write: "It has bees ascribed to Celtic influence, but this particular alternationdoes not cerrespond to any of the usual mutanional patterns in Celtic languages." Maybe it was a child who brought it up, maybe some other imaginative person, but this is just guessing.
Andy ;—)