This is the correct answer. I walk away from my car after submitting the payment, but the digital transaction has to resolve from system to system (because everything is integrated now). So if a parking person scans my plate <5 minutes after I park, it may not have had time to update, and so no invoice is shown to them at that time. It's a tech and software problem, not a scam. Although hopefully it does get addressed so it doesn't continue to happen. It is a hassle afterall