The difference can be with the pre-authorisation type of authorisations. This type of authorisation can be voided or finalised in a smaller amount than initial authorisation. For example, the initial amount was 10 euros, and the standing amount is 7 euros.
For Final-authorisation type it can be different if you make a refund and after that request status. You will see that initial_amount will be amount of initial payment amount and the standing amount can be 0 (if you made full refund) or some other amount (if you made partial refund).