The Default Approver is a user defined in an approval step to handle cases where no Approver is assigned based on the rules set in the approval matrix.
When the Default Approver approves a request, it does authorise that step.
However, the request will only be synced to the accounting system once all steps in the workflow have been approved, including those that do have designated Approvers.
If any step is still pending (e.g. another Approver has not approved yet), the request will remain in the Open or On Approval status in ApprovalMax until fully authorised.