It seems to have become the norm for young cashiers to be unable to understand. And if you try to explain, they'll insist "I can't change it now I've rung it through". Some seem to think the system keeps an exact record of the quantity of each individual coin (or they just don't even know where to begin to think about it).
It's like the difference between a language you are fluent in and a language you are tentative in. If you're fluent, you have to make an effort not to listen to somebody's loud conversation, or not to pay attention to a billboard. They intrude into your consciousness. There's never a situation when I don't do simple arithmetic when exposed to it. I don't have to consciously figure out what 4 times 9 is. Subjectively, the number just pops into my head when I see the question.
edit: If you can't do this with explanations of identities or related rates, etc., it's hard or impossible to follow any quantitative or especially probabilistic argument. Even the simplest ones. I think this results in people for whom arithmetic is difficult faking it by trying to memorize the words used during quantitative arguments without having any real understanding. Just sort of memorizing a lot of slogans and repeating them during any argument that shares similar words. I think discomfort with arithmetic ruins people politically (as citizens), so I really do think calculators are a problem.