There is a luck factor with any block discovered. Some blocks take far more hashing power to fully confirm and therefore have "bad" luck and requires more shares to complete. Others are quick and therefore have good "luck". No one can predict how much hashing power is going to be required until after the block has been completed. Since the block only pays out a specific number of coins blocks that require more hashing power are considered unlucky. Over the long term it averages out to make the 100%.