well, either way a different sensor is a different sensor. If it has a different sensitivity around the operating temp (temp sensors are supposed to have low sensitivity around operating temp and high sensitivity low and high so that small fluctuations around ideal temp wont show up but large ones will). My guess (and that's all it is) is that your sensor does not have the same (non) linearity as the old one. Can you swap the old sensor back in just to test this hypothesis?