you’re missing the forest due to the single example being given; kind of like their coworker missing the point about rounding. the lack of the fundamental understanding of mathematics is leading to this person making a mistake repeatedly and their insistence that they followed the rules (which they don’t actually understand don’t apply here) leads to confidently incorrect answers. these types of behaviors will show up repeatedly in many contexts.
its amusing how you can’t realize you’re demonstrating the same behavior as the coworker.