Previously the mag-stripe conducted your card number to the merchant and they could charge essentially whatever they wanted (but there were various reasons they would likely charge the amount you owed). With chip, they have to compute the final amount while your card is inserted and cannot deviate.
Place I part time at the card terminal isn't even hooked into the cash register. I have to manually enter the amount from the register screen into the card terminal. But I have to do that for mag stripe and contactless as well.
