Report - Aged Accounts Payable (Report ID 322)

We have been having problems with this report for many months. When we generated the report as at a certain date, we expected that the balance figures on the report would equal to the figures of adding up the Detailed Vendor Ledg. Entries up to the specified date. However, it wasn’t the case for some vendors and we had no idea why was that so. Apparently the report doesn’t work in this way (adding up the ledg. entries). Can anyone pls advise how does this Aged Accounts Payable report work? We are using Nav 3.70 (SG version). Thank you.