Variances between control accounts and Payable/Receivable Report

What is the real cause of the differences between the Control Account in GL and Payable/Receivable report. This is a serious issue we are always having in the system. many times our external auditor complained about this issues between the control account and subledger report not agreeing.

it is a rule that when capturing the invoice both the document date and posting date must be the same. 

What is really causing the difference between the two report?

thanks