The way Wal-Mart, Target and others work though isn't that you ever get to explain things.
Something scans wrong? Employee punches something in and you don't get wrung up for an item? Some AI camera system decides you've done some sort of switcheroo?
They will have the police come to arrest you next time you're at the store and have long since forgotten the details of your last purchase. Now your chance to explain involves lawyer fees - way more expensive than most credit card fees.
If you notice the mistake and try to explain to an employee before it's chargable as theft, and think you get it cleared up: there's still a good chance you still get charged and also the employee is fired and/or charged too. Major retailers assume their employees are just as much of a thief as they think you are.
if there is a mistake and someone thinks I stole something, I'd like to be able to explain that I did not, rather than having false charge on my card.