The difference could be to do with an unallocated Cash balance and an o/s bill or disbursement. This could also be to do with a reversal of a disbursement and an o/s bill for the same value.
Follow the below steps to run a report to find the matters with a zero-office balance but o/s bills and disbursements:
From the Main Menu select Advanced Report Execution.
Select the Accounts Standard folder.
Select the House Keeping folder.
Click on the report called Matters with Zero Office and Outstanding Balances.
Select either a Practice or untick for all.
Select ACCEPT.