Hours tracked were reported inaccurate

I pulled our company hours report on 3/18 for the pay period 3/4-3/17. At that time the report showed hours for each of our tracked employees. However, after I submitted these hours to our payroll processor, several employees notified me that they had been under paid. I double checked the report, and a later pull had different hours recorded for these employees. I need to understand how this happened.