Using a silver wire as the reference electrode is the most likely reason for the small shift in the peak potentials. A silver wire reference is what is called a pseudo-reference electrode because, unless there is an ion present in fixed concentration in the solution that poises the potential of the silver wire, such as Ag+, Cl-, Br-, etc., then the potential of the silver wire will depend on the composition of the solution. The way around this would be to dope the solution with a fixed amount of such a poising ion or use a reference electrode separated by a salt bridge from the test solution.

Thanks Prof. Richard, this is really helpful and I will try to find a reference for this explanation