For new fixed assets the most likely source is a difference between the original cost as entered in Fixes Assets and the purchase as entered in purchases against the code FA.
For a new fixed asset there should be the following entries:
- Fixed Assets - in the most appropriate group
- Purchases - using code FA
- Current Account (or other bank sheet) - using code CR for the payment
Check the FA Reconciliation sheet in Fixed Assets before trying to unpick the Trial Balance.