This relates to Ad Hoc approvals. 

When you send an invoice out for approval via the Ad hoc method, you are asked to “Choose the Outcome”.
If you select “Treat as complete”, it will mark the invoice as approved after it has completed.
If you do not select this checkbox, the Approval Icon will flash amber as it will assume that either more approvers need to get involved or it requires more attention on your part.
If you have forgotten to tick that box when you have sent it out for approval. You can mark the invoice as approved by either sending it back to the same approver again, this time with “Treat As Complete” selected. Or send it to yourself for approval with the “Treat As Complete” selected.
For obvious reasons, we do not give our users the ability to mark invoices as “Approved” without having it go through an approval process.