The voltage difference in a voltaic cell can be attributed to standard conditions not being met, including temperature, concentration, and pressure variations. Specifically, deviations from the ideal 25 degrees Celsius, 1M solution, and 1 atm pressure can impact the predicted voltage. Additionally, internal resistance within the cell can further reduce the operating voltage. These factors collectively explain the discrepancies observed in real-world applications compared to theoretical predictions. Understanding these reasons is crucial for accurate electrochemical analysis.