Expanded Trial Balance - How to calculate Begin Total Activity columns?

I am trying to validate a custom trial balance report that shows all the (foreign) currency balances as well as functional balance within a natural account.  Unfortunately, querying the GL_BALANCES table I cannot tie the Begin Total Activity debit or credit amount in the Expanded Trial Balance report to the GL_BALANCES table.  Is there a bug with Expanded Trial Balance?  Or, can someone tell me how to calculate the Begin Total Activity Debit and Begin Total Activity Credit amounts that appear in this report.