what could cause the difference in the actual number of mole and the expected number of mole in copper oxide?

Answer 1

You could have weighed the Cu or CuO wrong. Weighing Cu wrong changes mols Cu and weighing CuO wrong changes mols O since you used the difference between Cu mass and CuO mass. The Cu you used may have had an oxide coating on it so you didn't have a true weight of Cu. You could have formed some CuO and some Cu2O and your numbers were really an average.