Most of these are understaffed with overworked workers. Sales happen, they don’t update the system and then customers don’t catch the mistake.

I had it happen near me. I had an item that was 3/10 and I begrudgingly paid 10 dollars instead of the 4.75 for one. I get to the counter and it rang up at full price. Had to walk the clerk back to the shelf to show them. Came in two weeks later and the same thing happened.