You may be right about the the "p"s and "f"s, but I don't think that applies to "finial". According to Hyperdictionary "finial" is derived from the Latin "finire" (to finish). On the other hand "phenomenon" comes from the Greek. For some reason the Greek "f" is "phi", which is almost always transliterated as "ph". I suspect this is because at some time in the past there was an audible difference between an English "f" sound and the Greek equivalent that has since disappeared.