This happens when the analysis code was updated after the transaction was posted. Existing transactions are not automatically updated when an analysis code changes. Reselect the analysis code on the transaction to correct it:
Go to Creditors.
Select Enquiries.
Click Purchase Transactions to load the required transaction.
On the detail line, click the popover button and then click Edit Transaction Details.
Click the magnifying glass next to the Analysis Code field and reselect the analysis code.
Click Save.
Reselecting the analysis code prompts the system to update the nominal codes accordingly.