The reason is that the kosher piece spreads around the non-kosher piece.
 But if the non-salty kosher piece was on the bottom, even if the non-kosher fatty piece is on top it does not disqualify the meat more than the klipah. However, if they are both lean it is still assur to the extent of the klipah.
 It says over there that in a case of fatty meat, it makes no difference whether the salty piece is on top or bottom. It also says that we cannot properly differentiate between fatty and lean, and all these calculations should be against the sixty ratio.