Rational Inattention
People economise on what they bother to work out. A heuristic that is right most of the time gets kept, which means it is a heuristic you will not notice failing.
What it is
Attention is finite and checking has a cost. Working out the unit price on every line of a weekly shop is arithmetic done while holding a basket, and the rule that bigger is cheaper is right often enough to be worth the saving.
That is not a failure of reasoning. It is reasoning about how much reasoning to do.