When you scan the QR code at the end it will usually tell you just to pay, but sometimes it will ask you to wait for an attendant to check that you have scanned everything correctly.

Assuming everything was accurate, it then updates a ‘confidence’ score in the background for you - and if you always scan everything correctly the checks quickly become less frequent / almost non-existent. If you always scan everything wrong, the attendants will be asked to check it every time.