I certainly can't tell apart people with a 3 point IQ difference but as the article mentions it is possible that this kind of differences would accumulate over

I certainly can't tell apart people with a 3 point IQ difference but as the article mentions it is possible that this kind of differences would accumulate over time. For example the difference between an A and a B is pretty small on a case by case basis, but it's a big difference between a straight A student and B average student.