I think you are misunderstanding the comment you replied to. They are saying that whenever a customer at a grocery store checkout was charged an incorrect price, the price was always higher than the advertised price, never lower.

Yes, customers that notice it can have it corrected to the accurate, lower price. But the problem is that whenever there were "whoopsies", it's hard to just believe it was just a bug or well-intentioned mistake when there were never mistakes that would have resulted in a lower price.