The issue she had is not math it’s attention to detail. She misread the screen and was so disengaged that she didn’t notice a massive discrepancy. If she’s that distracted on a regular basis she was going to get fired soon.
The part of this I would ascribe to the education system is that often kids are not required to take any accountability or taught how to strengthen their deficits. They’re just passed on so no one has to deal with them. You even hear that attitude echoed from a 12 year old, she probably didn’t know how to do math as though that’s just inevitable. NTA