If the cell contains a salt bridge, that also contributes to the total cell voltage. Since ions will diffuse (unequally) from the solution into the surfaces on either side of the salt bridge, the solution interface on each end of the bridge will have a different voltage associated with it. It's difficult to estimate the size of the net 'liquid junction potential' contributed by the bridge. It can be positive or negative, and it happens to be very sensitive to pH (since hydrogen ions are better than most ions at diffusing into the bridge).
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