This is one of those words Nuncle was talking about in another thread. The B in subtle, which was there in the Latin, seems to have been dropped in the Old French and wasn't there when we got it from them in the days of Middle English. It was added back in when a bunch of busy-bodies decided that, since it was there in the Latin, we needed it in the English. The B in subtile appears never to have been lost in the Old French. There is a notable difference in meaning, subtile meaning 'thin, rarefied', but it does also have the same meaning as subtle, perhaps simply by confusion with subtle. They both come from the Latin subtīlis.