Not sure how accurate this is (especially since I haven't finished the book) but on page 207 of the Bands of Mourning someone says that mistings are one in a thousand which is drastically different from the one in fifty ratio. They also said that ferrings are a lot less common and that there have only been 3 known twin born crashers in history other than Waxillium Ladrian. Take all of this with a grain of salt though because it could be wildly inaccurate for all I know.