Impartiality in a jury trial context doesn't refer to the absence of biases (we all have them and truly getting rid of them is an impossible task), but a conscious effect by the jurors to discharge their office without prejudice, i.e., without allowing their biases to manifest as judgments independent of the evidence presented in court.

Put another way: there wouldn't be a point in deliberation if we didn't have biases. Our justice system trusts and (weakly) attempts to enforce the gap between those biases and prejudices that would actually interfere with the court's duty.